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The Bw-Tree：A B-tree for New Hardware
1 . The Bw-Tree: A B-tree for New Hardware Platforms Justin J. Levandoski 1 , David B. Lomet 2 , Sudipta Sengupta 3 Microsoft Research Redmond, WA 98052, USA 1 2 3 email@example.com, firstname.lastname@example.org, email@example.com Abstract— The emergence of new hardware and platforms has operations without setting latches. Our approach also carefully led to reconsideration of how data management systems are avoids cache line invalidations, hence leading to substantially designed. However, certain basic functions such as key indexed better caching performance as well. We describe how we use access to records remain essential. While we exploit the common architectural layering of prior systems, we make radically new our log structured storage manager at a high level, but leave design decisions about each layer. Our new form of B-tree, called the specifics to another paper. the Bw-tree achieves its very high performance via a latch-free approach that effectively exploits the processor caches of modern B. The New Environment multi-core chips. Our storage manager uses a unique form of log Database systems have mostly exploited the same storage structuring that blurs the distinction between a page and a record and CPU infrastructure since the 1970s. That infrastructure store and works well with flash storage. This paper describes the used disks for persistent storage. Disk latency is now analo- architecture and algorithms for the Bw-tree, focusing on the main memory aspects. The paper includes results of our experiments gous to a round trip to Pluto . It used processors whose that demonstrate that this fresh approach produces outstanding uni-processor performance increased with Moore’s Law, thus performance. limiting the need for high levels of concurrent execution on a single machine. Processors are no longer providing ever higher I. I NTRODUCTION uni-core performance. Succinctly, ”this changes things”. A. Atomic Record Stores 1) Design for Multi-core: We live in a high peak perfor- There has been much recent discussion of No-SQL systems, mance multi-core world. Uni-core speed will at best increase which are essentially atomic record stores (ARSs) . While modestly, thus we need to get better at exploiting a large some of these systems are intended as stand-alone products, number of cores by addressing at least two important aspects: an atomic record store can also be a component of a more 1) Multi-core cpus mandate high concurrency. But, as the complete transactional system, given appropriate control op- level of concurrency increases, latches are more likely erations , . Indeed, one can regard a database system as to block, limiting scalability . including an atomic record store as part of its kernel. 2) Good multi-core processor performance depends on high An ARS supports the reading and writing of individual CPU cache hit ratios. Updating memory in place results records, each identified by a key. Further, a tree-based ARS in cache invalidations, so how and when updates are supports high performance key-sequential access to designated done needs great care. subranges of the keys. It is this combination of random and Addressing the first issue, the Bw-tree is latch-free, ensuring a key-sequential access that has made B-trees the indexing method thread never yields or even re-directs its activity in the face of of choice within database systems. conflicts. Addressing the second issue, the Bw-tree performs However, an ARS is more than an access method. It includes “delta” updates that avoid updating a page in place, hence the management of stable storage and the requirement that preserving previously cached lines of pages. its updates be recoverable should the system crash. It is the 2) Design for Modern Storage Devices: Disk latency is a performance of the ARS of this more inclusive form that is major problem. But even more crippling is their low I/O ops the foundation for the performance of any system in which the per second. Flash storage offers higher I/O ops per second at ARS is embedded, including full function database systems. lower cost. This is key to reducing costs for OLTP systems. This paper introduces a new ARS that provides very high Indeed Amazon’s DynamoDB includes an explicit ability to performance. We base our ARS on a new form of B-tree that exploit flash . Thus the Bw-tree targets flash storage. we call the Bw-tree. The techniques that we introduce make Flash has some performance idiosyncracies, however. While the Bw-tree and its associated storage manager particularly ap- flash has fast random and sequential reads, it needs an erase cy- propriate for the new hardware environment that has emerged cle prior to write, making random writes slower than sequential over the last several years. writes . While flash SSDs typically have a mapping layer Our focus in this paper is on the main memory aspects of (the FTL) to hide this discrepancy from users, a noticeable the Bw-tree. We describe the details of our latch-free tech- slowdown still exists. As of 2011, even high-end FusionIO nique, i.e., how we can do updates and structure modification drives exhibit a 3x faster sequential write performance than
2 .random writes . The Bw-tree performs log structuring it- API Tree-based search/update logic self at its storage layer. This approach avoids dependence on Bw-tree Layer In-memory pages only the FTL and ensures that our write performance is as high Logical page abstraction for B-tree layer Cache Layer as possible for both high-end and low-end flash devices and Maintains mapping table, brings pages hence, not the system bottleneck. Mapping Table from flash to RAM as necessary Flash Layer Manages writes to flash storage C. Our Contributions Flash garbage collection We now describe our contributions, which are: Fig. 1. The architecture of our Bw-tree atomic record store. 1) The Bw-tree is organized around a mapping table that virtualizes both the location and the size of pages. This virtualization is essential for both our main memory II. B W- TREE A RCHITECTURE latch-free approach and our log structured storage. The Bw-tree atomic record store (ARS) is a classic B+- 2) We update Bw-tree nodes by prepending update deltas tree  in many respects. It provides logarithmic access to to the prior page state. Because our delta updating pre- keyed records from a one-dimensional key range, while pro- serves the prior page state, it improves processor cache viding linear time access to sub-ranges. Our ARS has a classic performance. Having the new node state at a new storage architecture as depicted in Figure 1. The access method layer, location permits us to use the atomic compare and swap our Bw-tree Layer, is at the top. It interacts with the middle instructions to update state. Hence, the Bw-tree is latch- Cache Layer. The cache manager is built on top of the Storage free in the classic sense of allowing concurrent access Layer, which implements our log-structured store (LSS). The to pages by multiple threads. LSS currently exploits flash storage, but it could manage with 3) We have devised page splitting and merging structure either flash or disk. modification operations (SMOs) for the Bw-tree. SMOs This design is architecturally compatible with existing are realized via multiple atomic operations, each of which database kernels, while also being suitable as a standalone leaves the Bw-tree well-formed. Further, threads observ- “data component” in a decoupled transactional system , ing an in-progress SMO do not block, but rather take . However, there are significant departures from this classic steps to complete the SMO. picture. In this section, we provide an architectural overview 4) Our log structured store (LSS), while nominally a page of the Bw-tree ARS, describing why it is uniquely well-suited store, uses storage very efficiently by mostly posting for multi-core processors and flash based stable storage. page change deltas (one or a few records). Pages are eventually made contiguous via consolidating delta up- A. Modern Hardware Sensitive dates, and also during a flash “cleaning” process. LSS In our Bw-tree design, threads almost never block. Elimi- will be described fully in a another paper. nating latches is our main technique. Instead of latches, we in- 5) We have designed and implemented an ARS based on stall state changes using the atomic compare and swap (CAS) the Bw-tree and LSS. We have measured its performance instruction. The Bw-tree only blocks when it needs to fetch using real and synthetic workloads, and report on its very a page from stable storage (the LSS), which is rare with a high performance, greatly out-performing both Berke- large main memory cache. This persistence of thread execution leyDB, an existing system designed for magnetic disks, helps preserve core instruction caches, and avoids thread idle and latch-free skip lists in main memory. time and context switch costs. Further, the Bw-tree performs In drawing broader lessons from this work, we believe that node updates via ”delta updates” (attaching the update to an latch free techniques and state changes that avoid update-in- existing page), not via update-in-place (updating the existing place are the keys to high performance on modern processors. page memory). Avoiding update-in-place reduces CPU cache Further, we believe that log structuring is the way to provide invalidation, resulting in higher cache hit ratios. Reducing high storage performance, not only with flash, but also with cache misses increases the instructions executed per cycle. disks. We think these “design paradigms” are applicable more Performance of data management systems is frequently widely to realize high performance data management systems. gated by I/O access rates. We have chosen to target flash storage to ease that problem. But even with flash, when D. Paper Outline attached as an SSD, I/O access rates can limit performance. We present an overview of Bw-tree architecture in Section 2. Our log structure storage layer enables writing large buffers, In Sections 3 through 5, we describe the system we built. effectively eliminating any write bottle neck. Flash storage’s We start at the top layer with in-memory page organization high random read access rates coupled with a large main in Section 3, followed by Bw-tree organization and structure memory cache minimizes blocking on reads. Writing large modifications in Section 4. Section 5 details how the cache is multi-page buffers permits us to write variable size pages that managed. In Section 6, we describe our experiments and the do no contain “filler” to align to a uniform size boundary. performance results of them. Section 7 describes related work The rest of this section summarizes the major architectural and how we differ significantly in our approach. We conclude and algorithmic innovations that make concrete the points de- with a short discussion in Section 8. scribed above.
3 .B. The Mapping Table D. Bw-tree Structure Modifications Our cache layer maintains a mapping table, that maps logi- Latches do not protect parts of our index tree during struc- cal pages to physical pages, logical pages being identified by a ture modifications (SMOs) such as page splits. This introduces logical ”page identifier” or PID. The mapping table translates a problem. For example, a page split introduces changes to a PID into either (1) a flash offset, the address of a page on more than one page: the original overly large page O, the new stable storage, or (2) a memory pointer, the address of the page page N that will receive half O′ s contents, and the parent in- in memory. The mapping table is thus the central location for dex page P that points down to O, and that must subsequently managing our “paginated” tree. While this indirection tech- point to both O and N . Thus, we cannot install a page split nique is not unique to our approach, we exploit it as the base with a single CAS. A similar but harder problem arises when for several innovations. We use PIDs in the Bw-tree to link the we merge nodes that have become too small. nodes of the tree. For instance, all downward “search” pointers To deal with this problem, we break an SMO into a sequence between Bw-tree nodes are PIDs, not physical pointers. of atomic actions, each installable via a CAS. We use a B-link The mapping table severs the connection between physical design  to make this easier. With a side link in each page, location and inter-node links. This enables the physical loca- we can decompose a node split into two “half split” atomic tion of a Bw-tree node to change on every update and every actions. In order to make sure that no thread has to wait for time a page is written to stable storage, without requiring that an SMO to complete, a thread that sees a partial SMO will the location change be propagated to the root of the tree (i.e., complete it before proceeding with its own operation. This updating inter-node links). This “relocation” tolerance directly ensures that no thread needs to wait for an SMO to complete. enables both delta updating of the node in main memory and log structuring of our stable storage, as described below. E. Log Structured Store Bw-tree nodes are thus logical and do not occupy fixed Our LSS has the usual advantages of log structuring . physical locations, either on stable storage or in main memory. Pages are written sequentially in a large batch, greatly reducing Hence we are free to mold them to our needs. A “page” for a the number of separate write I/Os required. However, because node thus suggests a policy, not a requirement, either in terms of garbage collection, log structuring normally incurs extra of how we represent nodes or how large they might become. writes to relocate pages that persist in reclaimed storage areas We permit page size to be elastic, meaning that we can split of the log. Our LSS design greatly reduces this problem. when convenient as size constraints do not impose a splitting When flushing a page, the LSS need only flush the deltas requirement. that represent the changes made to the page since its previous flush. This dramatically reduces how much data is written C. Delta Updating during a flush, increasing the number of pages that fit in the Page state changes are done by creating a delta record (de- flush buffer, and hence reducing the number of I/O’s per page. scribing the change) and prepending it to an existing page There is a penalty on reads, however, as the discontinuous parts state. We install the (new) memory address of the delta record of pages all must be read to return a page to the main memory into the page’s physical address slot in the mapping table cache. Here is when the very high random read performance using the atomic compare and swap (CAS) instruction1. If of flash really contributes to our ARS performance. successful, the delta record address becomes the new physi- The LSS cleans prior parts of flash representing the old parts cal address for the page. This strategy is used both for data of its log storage. Delta flushing reduces pressure on the LSS changes (e.g., inserting a record) and management changes cleaner by reducing the amount of storage used per page. This (e.g., a page being split or flushing a page to stable storage). reduces the “write amplification” that is a characteristic of log Occasionally, we consolidate pages (create a new page that structuring. During cleaning, LSS makes pages and their deltas applies all delta changes) to both reduce memory footprint and contiguous for improved access performance. to improve search performance. A consolidated form of the page is also installed with a CAS, and the prior page structure F. Managing Transactional Logs is garbage collected (i.e., its memory reclaimed). A reference As in a conventional database system, our ARS needs to to the entire data structure for the page, including deltas, is ensure that updates persist across system crashes. We tag each placed on a pending list all of which will be reclaimed when update operation with a unique identifier that is typically the safe. We use a form of epoch to accomplish safe garbage log sequence number (LSN) of the update on the transactional collection, . log (maintained elsewhere, e.g., in a transactional component). Our delta updating simultaneously enables latch-free access LSNs are managed so as to support recovery idempotence, i.e., in the Bw-tree and preserves processor data caches by avoiding ensuring that operations are executed at most once. update-in-place. The Bw-tree mapping table is the key enabler Like conventional systems, pages are flushed lazily while of these features via its ability to isolate the effects of node honoring the write-ahead log protocol (WAL). Unconvention- updates to that node alone. ally, however, we do not block page flushes to enforce WAL. 1 The CAS is an atomic instruction that compares a given old value to a Instead, because the recent updates are separate deltas from current value at memory location L, if the values are equal the instruction the rest of the page, we can remove “recent” updates (not yet writes a new value to L, replacing current. on the stable transactional log) from pages when flushing.
4 . LPID Ptr ∆D LPID Ptr Consolidated the current address in the mapping table. If the current address Page P equals P , the CAS successfully installs D, otherwise it fails P CAS P CAS Page P ∆ ∆ ∆ (we discuss CAS failures later). Since all pointers between Bw-tree nodes are via PIDs, the CAS on the mapping table Page P entry is the only physical pointer change necessary to install a (a) Update using delta record (b) Consolidating a page page update. Furthermore, this latch-free technique is the only way to update a page in the Bw-tree, and is uniform across Fig. 2. In-memory pages. Page updates use compare-and-swap (CAS) on all operations that modify a page. In the rest of this paper, the physical pointer in the mapping table. we refer to using the CAS to update a page as “installing” an operation. Figure 2(a) depicts the update process showing a delta record III. I N -M EMORY L ATCH F REE PAGES D prepended to page P . The dashed line from the mapping This section describes Bw-tree in-memory pages. We begin table to P represents P ’s old address, while the solid line to by discussing the basic page structure and how we update the delta record represents P ’s new physical address. Since pages in a latch-free manner. We then discuss occasional page updates are atomic, only one updater can “win” if there are consolidation used to make search more efficient. Using a competing threads trying install a delta against the same “old” tree based index enables range scans, and we describe how state in the mapping table. A thread must retry its update if it this is accomplished. Finally, we discuss in-memory garbage fails2 . collection with our epoch safety mechanism. After several updates, a “delta chain” forms on a page as depicted in the lower right of Figure 2(b) (showing a chain A. Elastic Virtual Pages of three deltas). Each new successful updates forms the new The information stored on a Bw-tree page is similar to that “root” of the delta chain. This means the physical structure of of a typical B+-tree. Internal index nodes contain (separator a Bw-tree page consists of a delta chain prepended to a base key,pointer) pairs sorted by key that direct searches down the page (i.e., a consolidate B-tree node). For clarity, we refer tree. Data (leaf) nodes contain (key, record) pairs. In addition, to a base page as the structure to which the delta chain is pages also contain (1) a low key representing the smallest key prepended, and refer to a page as the a base page along with value that can be stored on the page (and in the subtree below), its (possibly empty) delta chain. and (2) a high key representing the largest key value that can Leaf-level update operations. At the leaf page level, up- be stored on the page, and (3) a side link pointer that points dates (deltas) are one of three types: (1) insert, representing to the node’s immediate right sibling on the same level in the a new record inserted on the page; (2) modify, representing tree, as in a B-link tree . a modification to an existing record in the page; (3) delete, Two distinct features make the Bw-tree page design unique. representing the removal of an existing record in the page. All First, Bw-tree pages are logical, meaning they do not occupy update deltas contain an LSN provided by the client issuing the fixed physical locations or have fixed sizes. (1) We identify update. We use this LSN for transactional recovery involving a page using its PID index into the mapping table. Accessors a transaction log manager with its need to enforce the write- to the page use the mapping table to translate the PID into ahead-log (WAL) protocol (we discuss WAL and LSNs further its current physical address. All links between Bw-tree nodes in Section V-A). Insert and update deltas contain a record (“search” pointers, side links) are PIDs. (2) Pages are elastic, representing the new payload, while delete deltas only contain meaning there is no hard limit on how large a page may grow. the key of the record to be removed. In the rest of this sec- Pages grow by having “delta records” prepended to them. tion, we discuss only record updates to leaf pages, postponing A delta record represents a single record modification (e.g., discussion of other updates (e.g., index pages, splits, flushes) insert, update), or system management operation (e.g., page until later in the paper. split). Page search. Leaf page search involves traversing the delta Updates. We never update a Bw-tree page in place (i.e., chain (if present). The search stops at the first occurrence of modify its memory contents). Rather, we create a delta record the search key in the chain. If the delta containing the key describing the update and prepend it to the existing page. represents an insert or update, the search succeeds and returns Delta records allow us to incrementally update page state in the record. If the delta represents a delete, the search fails. If a latch-free manner. We first create a new delta record D that the delta chain does not contain the key, the search performs (physically) points to the page’s current address P . We obtain a binary search on the base page in typical B-tree fashion. We P from the page’s entry in the mapping table. The memory discuss index page search with delta chains in Section IV. address of the delta record will serve as the new memory B. Page Consolidation address (the new state) for the page. To install the new page state in the mapping table (making Search performance eventually degrades if delta chains grow it “live” in the index), we use the atomic compare and swap too long. To combat this, we occasionally perform page con- (CAS) instruction (described in Section II) to replace the cur- 2 The retry protocol will depend on the specific update operation. We discuss specific rent address P with the address of D. The CAS compares P to retry protocols where appropriate.
5 .solidation that creates a new “re-organized” base page contain- LPID Ptr LPID Ptr ing all the entries from the original base page as modified by O O Split ∆ P P CAS the updates from the delta chain. We trigger consolidation if Page P Page R Page P Page R an accessor thread, during a page search, notices a delta chain Q Page Q Q Page Q length has exceeded a system threshold. The thread performs consolidation after attempting its update (or read) operation. When consolidating, the thread first creates a new base page (a) Creating sibling page Q (b) Installing split delta (a new block of memory). It then populates the base page with Index entry ∆ LPID Ptr a sorted vector containing the most recent version of a record O CAS from either the delta chain or old base page (deleted records P Split ∆ are discarded). The thread then installs the new address of Q Page P Page R the consolidated page in the mapping table with a CAS. If Page Q it succeeds, the thread requests garbage collection (memory reclamation) of the old page state. Figure 2(b) provides an (c) Installing index entry delta example depicting the consolidation of page P that incorpo- Fig. 3. Split example. Dashed arrows represent logical pointers, while solid rates deltas into a new “Consolidated Page P ”. If this CAS arrows represent physical pointers. fails, the thread abandons the operation by deallocating the new page. The thread does not retry, as a subsequent thread deallocate the old page state while another thread still accesses will eventually perform a successful consolidation. it. Similar concerns arise when a page is removed from the Bw-tree. That is, other threads may still be able to access C. Range Scans the now removed page. We must protect these threads from A range scan is specified by a key range (low key, high accessing reclaimed and potentially “repurposed” objects by key). Either of the boundary keys can be omitted, meaning that preventing reclamation until such access is no longer possible. one end of the range is open-ended. A scan will also specify This is done by a thread executing within an “epoch”. either an ascending or descending key order for delivering the An epoch mechanism is a way of protecting objects be- records. Our description here assumes both boundary keys are ing deallocated from being re-used too early . A thread provided and the ordering is ascending. The other scan options joins an epoch when it wants to protect objects it is using are simple variants. (e.g., searching) from being reclaimed. It exits the epoch when A scan maintains a cursor providing a key indicating how this dependency is finished. Typically, a thread’s duration in far the search has progressed. For a new scan, the remembered an epoch is for a single operation (e.g. insert, next-record). key is lowkey. When a data page containing data in the range Threads “enrolled” in epoch E might have seen earlier ver- is first accessed, we construct a vector of records containing sions of objects that are being deallocated in epoch E. How- all records on the page to be processed as part of the scan. ever, a thread enrolled in epoch E cannot have seen objects In the absence of changes to the page during the scan, this deallocated in epoch E-1 because it had not yet started its allows us to efficiently provide the “next-record” functionality. dependency interval. Hence, once all threads enrolled in epoch This is the common case and needs to be executed with high E have completed and exited the epoch (“drained”), it is safe performance. to recycle the objects deallocated in epoch E. We use epochs We treat each ”next-record” operation as an atomic unit. The to protect both storage and deallocated PIDs. Until the epoch entire scan is not atomic. Transactional locking will prevent has drained such objects cannot be recycled. modifications to records we have seen (assuming serializable IV. B W- TREE S TRUCTURE M ODIFICATIONS transactions) but we do not know the form of concurrency control used for records we have not yet delivered. So before All Bw-tree structure modification operations (SMOs) are delivering a record from our vector, we check whether an performed in a latch-free manner. To our knowledge, this has update has affected the yet unreturned subrange in our record never been done before, and is crucial for our design. We first vector. If such an update has occurred, we reconstruct the describe node splits, followed by node merges. We then dis- record vector accordingly. cuss how to ensure an SMO has completed prior to performing actions that depend on the SMO. This is essential both to avoid D. Garbage Collection confusion in main memory and to prevent possible corrupt A latch-free environment does not permit exclusive access trees should the system crash at an inopportune time. to shared data structures (e.g., Bw-tree pages), meaning one or more readers can be active in a page state even as it is being A. Node Split updated. We do not want to deallocate memory still accessed Splits are triggered by an accessor thread that notices a page by another thread. For example, during consolidation, a thread size has grown beyond a system threshold. After attempting “swaps out” the old state of a page (i.e., delta chain plus base its operation, the thread performs the split. page) for a new consolidated state and requests that the old The Bw-tree employs the B-link atomic split installation state be garbage collected. However, we must take care not to technique that works in two phases . We first atomically
6 .install the split at the child (e.g., leaf) level. This is called a must now traverse a delta chain on the index nodes, finding half split. We then atomically update the parent node with the a boundary key delta in the chain such that a search key v new index term containing a new separator key and a pointer is greater than KP and less than or equal to KQ allows the to the newly created split page. This process may continue re- search to end instantly and follow the logical pointer down to cursively up the tree as necessary. The B-link structure allows Q. Otherwise, the search continues into the base page, which is us to separate the split into two atomic actions, since the side searched with a simple binary search to find the correct pointer link provides a valid search tree after installing the split at the to follow. Figure 3(c) depicts our running split example after child level. prepending the index entry delta to parent page O, where the Child Split. To split a node P , the B-tree layer requests dashed line represents the logical pointer to page Q. (from the cache layer) allocation for a new node Q in the Consolidations. Posting deltas decreases latency when in- mapping table (Q is the new right sibling of P ). We then stalling splits, relative to creating and installing completely find the appropriate separator key KP from P that provides a new base pages. Decreasing latency decreases the chance of balanced split and proceed to create a new consolidated base “failed splits”, i.e., the case that other updates sneak in before state for Q containing the records from P with keys greater we try to install the split (and fail). However, we eventually than KP . Page Q also contains a side link to the former right consolidate split pages at a later point in time. For pages with sibling of P (call this page R). We next install the physical split deltas, consolidation involves creating a new base page address of Q in the mapping table. This installation is done containing records with keys less than the separator key in the without a CAS, since Q is visible to only the split thread. split delta. For pages with index entry deltas, we create a new Figure 3(a) depicts this scenario, where a new sibling page consolidated base page containing the new separator keys and Q contains half the records of P , and (logically) points to pointers. page R (the right sibling of P ). At this point, the original (unsplit) state of P is still present in the mapping table, and B. Node Merge Q is invisible to the rest of the index. Like splits, node merges are triggered when a thread en- We atomically install the split by prepending a split delta counters a node that is below some threshold size. We peform record to P . The split delta contains two pieces of information: the node merge in a latch-free manner, which is illustrated in (1) the separator key KP used to invalidate all records within Figure 4. Merges are decidedly more complicated than splits, P greater than KP , since Q now contains these records and and we need more atomic actions to accomplish them. (2) a logical side pointer to the new sibling Q. This installation Marking for Delete. The node R to be merged (to be completes the first “half split”. Figure 3(b) depicts such a removed) is updated with a remove node delta, as depicted scenario after prepending a split delta to page P pointing to in Figure 4(a). This stops all further use of node R. A thread its new sibling page Q. At this point, the index is valid, even encountering a remove node delta in R needs to read or update without the presence of an index term for Q in the parent node the contents of R previously contained in R by going to the O. All searches for a key contained within Q will first go to left sibling, into which data from R will be merged. P . Upon encountering the split delta on P , the search will Merging Children.The left sibling L of R is updated with a traverse the side link to Q when the search key is greater than node merge delta that physically points (via a memory address) separator key KP . Meanwhile, all searches for keys less than to the contents of R. Figure 4(b) illustrates what L and R the KP remain at P . look like during a node merge. Note that the the node merge Parent Update. In order to direct searches directly to Q, delta indicates that the contents of R are to be included in L. we prepend an index term delta record to the parent of P and Further, it points directly to this state, which is now logically Q to complete the second half split. This index delta contains considered to be part of L. This storage for R’s state is now (1) KP , the separator key between P and Q, (2) a logical transferred to L (except for the remove node delta itself). It pointer to Q, and (3) KQ , the separator key for Q (formerly will only be reclaimed when L is consolidated. This turns what the separator directing searches to P ). We remember our path had previously been a linear delta chain representing a page down the tree (i.e. the PIDs of nodes on the path) and hence state into a tree. can immediately identify the parent. Most of the time, the When we search L (now responsible for containing both remembered parent on the path will be the correct one and we its original key space and the key space that had been R’s) do the posting immediately. Occasionally the parent may have the search becomes a tree search which directs the accessing been merged into another node. But our epoch mechanism thread to either L’s original page or to the page that it sub- guarantees that we will see the appropriate deleted state that sumed from R as a result of the merge. To enable this, the will tell us this has happened. That is, we are guaranteed that node merge delta includes the separator key that enables the the parent PID will not be a dangling reference. When we search to proceed to the correct node. detect a deleted state, we go up the tree to the grandparent Parent Update. The parent node P of R is now updated node, etc., and do a re-traversal down the tree to find the by deleting its index term associated with R, shown in Fig- parent that is “still alive”. ure 4(c). This is done by posting an index term delete delta Having KP and KQ present in the boundary key delta that includes not only that R is being deleted, but also that L is an optimization to improve search speed. Since searches will now include data from the key space formerly contained
7 .LPID Ptr Remove LPID Ptr Remove it must complete the split SMO by posting the new index term P Node ∆ P Merge ∆ Node ∆ L L delta to the parent. Only then can it continue on to its own R Page L Page R Page S R Page L Page R Page S S S activity. That forces the incomplete SMO to be “committed” and to serialize before the interrupted action the thread had (a) Posting remove node delta (b) Posting merge delta started upon. ∆ Delete Index Term for R The same principle applies regardless of whether the SMO is a split or a node merge. When deleting a node R, we LPID Ptr P Merge ∆ Remove access its left sibling L to post the merge delta. If we find Node ∆ L Page L Page R Page S that L is being deleted, we are seeing an in progress and R S incomplete “earlier” system transaction. We need the delete of R to serialize after the delete of L in this case. Hence (c) Posting index term delete delta the thread deleting R needs to first complete the delete of Fig. 4. Merge example. Dashed arrows represent logical pointers, while solid L. Only then can this thread complete the delete of R. All arrows represent physical pointers. combinations of SMO are serialized in the same manner. This can lead to the processing of a “stack” of SMOs, but given the rarity of this situation, it should not occur often, and is by R. The new range for L is explicitly included with a low reasonably straightforward to implement recursively. key equal to L’s prior low key and a high key equal to R’s prior high key. As with node splits, this permits us to recognize V. C ACHE M ANAGEMENT when a search needs to be directed to the newly changed part The cache layer is responsible for reading, flushing, and of the tree. Further, it enables any search that drops through swapping pages between memory and flash. It maintains the all deltas to the base page to find the right index term by a mapping table and provides the abstraction of logical pages simple binary search. to the Bw-tree layer. When the Bw-tree layer requests a page Once the index term delete delta is posted, all paths to R reference using a PID, the cache layer returns the address in are blocked. At this point we initiate the process of reclaiming the mapping table if it is a memory pointer. Otherwise, if Rs PID. This involves posting the PID to the pending delete the address is a flash offset (the page is not in memory), it list of PIDs for the currently active epoch. R’s PID will not reads the page from the LSS to memory, installs the memory be recycled until all other threads that might have seen an address in the mapping table (using a CAS), and returns the earlier state of R have exited the epoch. This is the same epoch new memory pointer. All updates to pages, including those mechanism we use to protect page memory from premature involving page management operations like split and page recycling (Section III-D). flush involve CAS operations on the mapping table at the location indexed by the PID. C. Serializing Structure Modifications and Updates Pages in main memory are occasionally flushed) to stable Our Bw-tree implementation assumes that conflicting data storage for a number of reasons. For instance, the cache layer update operations are prevented by concurrency contol that is will flush updates to enable the checkpointing of a transac- elsewhere in the system. This could be in the lock manager tional log when the Bw-tree is part of a transactional system. of an integrated database system, a transactional component Page flushes also precede a page swapout, installing a flash of a decoupled transactional system (e.g., Deuteronomy ), offset in the mapping table and reclaiming page memory in or finally, an arbitrary interleaving of concurrent updates as order to reduce memory usage. With multiple threads flush- enabled as by an atomic record store. ing pages to flash, multiple page flushes that need a correct However, “inside” the Bw-tree, we need to correctly seri- ordering must be done very carefully. We describe one such alize data updates with SMOs and SMOs with other SMOs. scenario for node splits in this section. That is, we must be able to construct a serial schedule for To keep track of which version of the page is on stable everything that occurs in the Bw-tree, where data updates and storage (the LSS) and where it is, we use a flush delta record, SMOs are treated as the units of atomicity. This is nontrivial. which is installed at the mapping table entry for the page We want to treat an SMO as atomic (think of them as system using a CAS. Flush delta records also record which changes transactions), and we are doing this without using latches that to a page have been flushed so that subsequent flushes send could conceal the fact that there are multiple steps involved in only incremental page changes to stable storage. When a page an SMO. One way to think about this is that if a thread stum- flush succeeds, the flush delta contains the new flash offset bles upon an incomplete SMO, it is like seeing uncommitted and fields describing the state of the page that was flushed. state. Being latch-free, the Bw-tree cannot prevent this from The rest section focuses on how the cache layer interacts happening. Our response is to require that such a thread must with the LSS layer by preparing and performing page flushes complete and commit the SMO it encounters before it can (LSS details covered in future work). We start by describing either (1) post its update or (2) continue with its own SMO. how flushes to LSS are coordinated with the demands of a For page splits, that means that when an updater or another separate transactional mechanism. We then describe the me- SMO would traverse a side pointer to reach the correct page, chanics of how flushes are executed.
8 .A. Write Ahead Log Protocol and LSNs B. Flushing Pages to the LSS The Bw-tree is an ARS that can be included in a transac- The LSS provides a large buffer into which the cache man- tional system. When included, it needs to manage the trans- ager posts pages and system transactions describing Bw-tree actional aspects imposed on it. The Deuteronomy  archi- structure modifications. We give a brief overview of this func- tecture makes those aspects overt as part of its protocol be- tionality. Details will be described in another paper. tween transactional (TC) and data component (DC), so we Page Marshalling. The cache manager marshalls the bytes use Deuteronomy terminology to describe this functionality. from the pointer representation of the page in main memory Similar considerations apply in other transactional settings. into a linear representation that can be written to the flush buffer. The page state is captured at the time it is intended to LSNs. Record insert and update deltas in a the Bw-tree page be flushed. This is important as later updates might violate the are tagged with the Log Sequence Number (LSN) of their op- WAL protocol or a page split may have removed records that erations. The highest LSN among updates flushed is recorded need to be captured in LSS. For example, the page may be split in the flush delta describing the flush. LSNs are generated by and consolidated while an earlier flush request for it is being the higher-level TC and are used by its transactional log. posted to the flush buffer. If the bytes for the earlier flush are Transaction Log Coordination. Whenever the TC appends marshalled after the split has removed the upper order keys in (flushes) to its stable transactional log, it updates the End of the pre-split page, the version of the page captured in the LSS Stable Log (ESL) LSN value. ESL is an LSN such that all will not have these records. Should the system crash before lower valued LSNs are definitely in the stable log. Periodically, the rest of the split itself is flushed, those records will be lost. it sends an updated ESL value to the DC. It is necessary for When marshalling records on a page for flush, multiple delta enforcing causality via the write-ahead log protocol (WAL) records are consolidated so that they appear contiguously in that the DC not make durable any operation with an LSN the LSS. greater than the latest ESL. This ensures that the DC is “run- Incremental Flushing. When flushing a page, the cache man- ning behind” the TC in terms of operations made durable. To ager only marshals those delta records which have an LSN enforce this rule, records on a page that have LSNs larger than between the previously flushed largest LSN on that page and the ESL are not included in a page when flushed to the LSS. the current ESL value. The previously flushed largest LSN Page flushes in the DC are required by the TC when it ad- information is contained in the latest flush delta record on the vances its Redo-Scan-Start-Point (RSSP). When the TC wishes page (as described above). to advance the RSSP, it proposes an RSSPit to the DC. The Incremental flushing of pages means that the LSS consumes intent is for this to permit the TC to truncate (drop) the portion much less storage for a page than is the case for full page of the transactional log earlier than the RSSP. The TC will flushing. This is very valuable for a log structured store such then wait for an acknowledgement from the DC indicating that as our LSS for two reasons. (1) It means that a flush buffer the DC has made every update with an LSN < RSSP stable. can hold far more updates across different pages than if the Because these operations results are stable, the TC no longer entire state of every page were flushed. This increases the has to send these operations to the DC during redo recovery. writing efficiency on a per page basis. (2) The log structured For the DC to comply, it needs to flush the records on every store cleaner (garbage collector) does not need to work as page that have LSNs < RSSP before it acknowledges to the hard since storage is not being consumed as fast. This reduces TC. Such flushes are never blocked as the cache manager can the execution cost per page for the cleaner. It also reduces exclude every record with an LSN > ESL. Non-blocking is not the ”write amplification”, i.e. the requirement of re-writing possible with a conventional update-in-place approach using a unchanged pages when the cleaner encounters them. single page-wide LSN . Flush Activity. The flush buffer aggregates writes to LSS up To enable this non-blocking behavior, we restrict page con- to a configurable threshold (currently set at 1MB) to reduce solidation (Section III-B) to include only update deltas that I/O overhead. It uses ping-pong (double) buffers and alternates have LSNs less than or equal to the ESL. Because of this, we between them with asynchronous I/O calls to the LSS so that can always exclude these deltas when we flush the page. the buffer for the next batch of page flushes can be prepared Bw-tree Structure Modifications. We wrap a system trans- while the current one is in progress. action  around the pages we log as part of Bw-tree SMOs. After the I/O completes for a flush buffer, the states of the This solves the problem of concurrent SMOs that can result respective pages are updated in the mapping table. The result from our latch-free approach (e.g., two threads trying to split of the flush is captured in the mapping table by means of a the same page). To keep what is in the LSS consistent with flush delta describing the flush, which is prepended to the state main memory, we do not commit the SMO system transaction and installed via a CAS like any other delta. If the flush has with its updated pages until we know that its thread has “won” captured all the updates to the page, the page is “clean” in the race to install an SMO delta record at the appropriate page. that there are no uncaptured updates not stable on the LSS. Thus, we allow concurrent SMOs, but ensure that at most one The cache manager monitors the memory used by the Bw- of them can commit. The begin and end system transaction tree, and when it exceeds a configurable threshold, it attempts records are among the very few objects flushed to the LSS to swap out pages to the LSS. Once a page is clean, it can be that are not instances of pages. evicted from the cache. The storage for the state of an evicted
9 . Synthetic Xbox Failed Splits Failed Consolidates Failed Updates 12 Dedup 0.25% 1.19% 0.0013% Operations/Second (M) 10 Xbox 1.27% 0.22% 0.0171% Synthetic 8.88% 7.35% 0.0003% 8 6 TABLE I L ATCH - FREE DELTA UPDATE FAILURES 4 2 It decides to install an element at the next highest layer (the 2 4 0 8 16 32 skip list towers or “express lanes”) with a probability of 21 . Delta Chain Length The maximum height of our skip list is 32 layers. Experiment machine. Our experiment machine is an Intel Fig. 5. Effect of Delta Chain Length Xeon W3550 (at 3.07GHz) with 24 GB of RAM. The machine page is reclaimed via our epoch-based memory garbage col- contains four cores that we hyperthread to eight logical cores lector (Section III-D). in all of our experiments. Evaluation Datasets. Our experiments use three workloads, VI. P ERFORMANCE E VALUATION two from real-world applications and one synthetic. This section provides experimental evaluation of the Bw- 1) Xbox LIVE. This workload contains 27 Million get-set tree when compared to a “traditional” B-tree architecture (Berke- operations obtained from Microsoft’s Xbox LIVE Prime- leyDB run in B-tree mode) as well as latch-free skiplists. Our time online multi-player game . Keys are alpha- experiments use a mix of real-world and synthetic workloads numeric strings averaging 94 bytes with payloads av- running on real system implementations. Since this paper fo- eraging 1200 bytes. The read-to-write ratio is approxi- cuses on in-memory aspects of the Bw-tree, our experiments mately 7.5 to 1. explicitly focus on in-memory system performance. Perfor- 2) Storage deduplication trace. This workload comes from mance evaluation on secondary storage will be the topic of a real enterprise deduplication trace used to generate future work. a sequence of chunk hashes for a root file directory A. Implementation and Setup and compute the number of deduplicated chunks and storage bytes. This trace contains 27 Million total chunks Bw-Tree. We implemented the Bw-Tree as a and 12 Million unique chunks, and has a read to write standalone atomic record store in approximately ration of 2.2 to 1. Keys are 20-byte SHA-1 hash values 10,000 lines of C++ code. We use the Win32 native that uniquely identify a chunk, while the value payload InterlockedCompareExchange64 to perform the CAS contains a 44-byte metadata string. update installation. Our entire implementation was latch-free. 3) Synthetic. We also use a synthetic data set that generates BerkeleyDB. We compare the Bw-tree to the BerkeleyDB 8-byte integer keys with a 8-byte integer payload. The key-value database. We chose BerkeleyDB due to its good workload begins with an index of 1M entries generated performance as a standalone storage engine. Furthermore, it using a uniform random distribution. It performs 42 mil- does not need to traverse a query processing layer as done in lion operations with a read to write ratio of 5 to 1. a complete database system. We use the C implementation of BerkeleyDB running in B-tree mode, which is a standalone B- Defaults. Unless mentioned otherwise, our primary perfor- tree index sitting over a buffer pool cache that manages pages, mance metric is throughput measured in (Million) operations representing a typical B-tree architecture. We use BerkeleyDB per second. We use 8 worker threads for each workload, equal in non-transactional mode (meaning better performance) that to the number of logical cores on our experiment machine. supports a single writer and multiple readers using page-level The default page size for both BerkeleyDB and the Bw-tree latching (the lowest latch granularity in BerkeleyDB) to max- is 8K (the skip list is a linked list and does not use page imize concurrency. With this configuration, we believe Berke- organization). leyDB represents a fair comparison to the Bw-tree. For all B. Bw-Tree Tuning and Properties experiments, the BerkeleyDB buffer pool size is large enough to accommodate the entire workload (thus does not touch In this section, we evaluate two aspects of the Bw-tree: secondary storage). (1) the effect of delta chain length on performance and (2) the Skip list. We also compare the Bw-tree to a latch-free skip latch-free failure rate for posting delta updates. list implementation . The skip list has become a popular 1) Delta chain length: Figure 5 depicts the performance alternative to B-trees for memory-optimized databases3 since of the Bw-tree run over the Xbox and synthetic workloads they can be implemented latch-free, exhibit fast insert perfor- for varying delta chain length thresholds, i.e., the maximum mance, and maintain logarithmic search cost. Our implemen- length a delta chain grows before we trigger page consol- tation installs an element in the bottom level (a linked list) idation. For small lengths, worker threads perform consol- using a CAS to change the pointer of the preceding element. idation frequently. This overhead cuts into overall system performance. For the Xbox workload, search deteriorates for 3 The MemSQL in-memory database uses a skip list as its ordered index  sequential scans larger than four deltas. While sequential scans
10 . BW-Tree BerkeleyDB Bw-tree Skip List 12.0 Synthetic workload 3.83M ops/sec 1.02M ops/sec 10.40 Read-only workload 5.71M ops/sec 1.30M ops/sec Operations/Sec (M) 10.0 TABLE II 8.0 B W- TREE AND LATCH - FREE SKIP LIST 6.0 3.83 In general, we believe two main aspects lead to the supe- 4.0 rior performance of the Bw-tree: (1) Latch-freedom: no thread 2.84 2.0 blocks on updates or reads on the Bw-tree, while BerkeleyDB 0.66 0.56 0.33 uses page-level latching to block readers during updates, re- 0.0 ducing concurrency. The Bw-tree executes with a processor Xbox Synthetic Deduplication utilization of about 99% while BerkeleyDB runs at about 60%. Fig. 6. Bw-tree and BerkeleyDB (2) CPU cache efficiency: since the Bw-tree uses delta records to update immutable base pages, the CPU caches of other over linked delta chains are good for branch prediction and threads are rarely invalidated on an update. Meanwhile, Berke- prefetching in general, the Xbox workload has large 100-byte leyDB updates pages in place. An insert into a typical B- records, meaning fewer deltas will fit into the L1 cache during tree page involves including a new element in vector of key- a scan. The synthetic workload contains small 8-byte keys, ordered records, on average moving half the elements and which are more amenable to prefetching and caching. Thus, invalidating multiple cache lines. delta chain lengths can grow longer (to about eight deltas) without performance consequences. D. Comparing Bw-tree to a Latch-Free Skip List 2) Update Failure in Latch-Free Environment: Given the We also compare the performance of the Bw-tree to a latch- latch-free nature of the Bw-tree, some operations will inevitably free skip list implementation. The skip list provides key-ordered fail in the race to update page state. Table I provides the access with logarithmic search overhead, and can easily be failure rate for splits, consolidates, and record updates for implemented in a latch-free manner. For these reasons, it is each workload. The record update failure rate (e.g., inserts, starting to receive attention as a B-tree alternative in memory- updates, deletes) is extremely low, below 0.02% for all work- optimized databases . The first row of Table II reports the loads. Meanwhile, the failure rates for the split and consoli- results of the synthetic workload run over both the Bw-tree date operations are larger than the update failures at around and the latch-free skiplist. The Bw-tree outperforms the skip 1.25% for both the Xbox and deduplication workloads, and list by a factor of 3.7x. To further investigate the source of the 8.88% for the synthetic workload. This is expected, since Bw-tree performance gain, we ran both systems using a read- splits and consolidates must compete with the faster record only workload that performs 30M key lookups on an index update operations. However, we believe these rates are still of 30M records. We report these results in the second row of manageable. The synthetic workload failure rates represent Table II, showing the Bw-tree with a 4.4x performance advan- a worst-case scenario. For the synthetic data, preparing and tage. These results suggest the Bw-tree has a clear advantage installing a record update delta is extremely fast since records in search performance. We suspect this is due to the Bw-tree are extremely small. In this case, a competing (and more having better CPU cache efficiency than the skip list, which expensive) split operation trying to install its delta on the same we explore in the next section. page has a high probability of failure. E. Cache Performance C. Comparing Bw-tree to a Traditional B-tree Architecture This experiment measures the CPU cache efficiency of the Bw-tree compared to the skip list. We use the Intel VTune This experiment compares the in-memory performance of profiler5 to capture CPU cache hit events while running the the Bw-tree to BerkeleyDB4 , representing a traditional B- read-only workload on each system. The workload was run tree architecture. Figure 6 reports the results for the Xbox, single-threaded in order to collect accurate statistics. Figure 7 deduplication, and synthetic workloads run on both systems. plots the distribution of retired memory loads over the CPU For the Xbox workload, the Bw-tree exhibits a throughput of cache hierarchy for each system. As we expected, the Bw-tree 10.4M operations/second, while BerkeleyDB has a throughput exhibits very good cache efficiency compared to the skip list. of 555K operations/second, representing a speedup of 18.7x. Almost 90% of its memory reads come from either the L1 The throughput of both systems drops for the update-intensive or L2 cache, compared to 75% for the skip list. We suspect deduplication workload, however the Bw-tree maintains a 8.6x the Bw-tree’s search efficiency is the primary reason for this speedup over BerkeleyDB. The performance gap is closer difference. The skip list must traverse a physical pointer before for the synthetic workload (5.8x Bw-tree speedup) due to every comparison to traverse to the next “tower” or linked list the higher latch-free failure rates observed for the Bw-tree record at a particular level. This can cause erratic CPU cache observed in Section VI-B.2. behavior, especially when branch prediction fails. Meanwhile, 4 We use BerkeleyDB’s memp stat function to ensure it runs in memory 5 http://software.intel.com/en-us/articles/intel-vtune-amplifier-xe/
11 . L1 hits L2 hits L3 hits RAM 100% Transactional log records, if they exist at all, are elsewhere. 90% We gain our latch-free capability from these deltas as well. 80% We have treated our flash SSD as a generic storage device. 70% Hence, we have not tried to exploit parallelism within it, as 60% done by . Exploiting this parallelism might well provide 50% even higher performance than we have achieved thus far. 40% 30% Log structuring was first applied to file systems . But 20% the approach is now more widely used for both disks and as 10% the translation layer for flash. In our system, we use solid state 0% disks, i.e. flash based devices that are accessed via the tradi- Bw-tree Skiplist tional I/O interface and use a translation layer. Despite this, Fig. 7. Cache efficiency we implemented our own log structured store. This enabled us to pack pages together in our buffer so that no empty space Bw-tree search is cache-friendly since most of its time is spent exists on flash. Further, it permitted us to blur the distinction performing binary search on a base page of keys compacted between a page store and a record store. This reduces even into a contiguous memory block (a few deltas may be present further the storage consumed in flushing pages. Storage sav- as well). This implies the Bw-tree will do much less pointer ings improve LSS performance as garbage collection, which chasing than the skip list. consumes cycles, amplifies writes, and wears out flash, is the The cache friendliness of the Bw-tree becomes clearer when largest cost in log structuring. we consider the set of nodes accessed in each data structure up to the point of reaching the data page. Over all possible D. Combination Efforts search pathways, this is precisely the set of internal nodes in the Bw-tree, which is 1% or less of the occupied memory. For Earlier indexes have used some of the elements above, though the skip list, this is about 50% of the nodes. So, for a given not in exactly the same way. Hyder  uses a log structured cache size, more of the memory accesses involved in a search store built on flash memory. In Hyder, all changes propagate hit the cache for Bw-tree over skip list. to the root of the tree. The changes at the root are batched, and the paths being included are compressed. Hyder, at least in one VII. R ELATED W ORK mode, supports transactions directly, with their log structured We have benefitted from prior work and in some cases have store used as both the database and the log. We do not do that, built upon it. relying instead on a transactional component when transaction support is desired. A. B-trees BFTL  is another example of an index implemented Anyone working with B-trees owes a debt to Bayer and over log structured flash. It is perhaps the closest to the Bw- McCreight . The variant that we use is the B+tree . We tree among earlier work in how it’s b-tree manages storage. It stretch the meaning of “page-oriented” by exploiting elastic has a mapping table, called a node translation table, and writes pages, in discontiguous pieces. The notion of elastic pages deltas (in our terminology) to flash. It also has a process for evolved from the hashed access method record lists used by making the deltas contiguous on flash when the number of SkimpyStash . Like SkimpyStash, we consolidate the dis- deltas gets large. However, the BTFL b-tree does not handle contiguous pieces of a “page” when we do garbage collection. multi-threading, concurrency control and cache management, B. Latch Free topics that we view as crucial to providing high throughput performance for an atomic record store. Until our work, it was not clear that B-trees could be made lock or latch-free. Skip lists  serve as an alternative latch- free “tree” when multiple threads must access the same page. VIII. D ISCUSSION Partitioning is another way to avoid latches so that each thread A. Performance Results has its own partition of the key space , . The Bw-tree avoids the overhead of partitioning by using the compare and Our Bw-tree implementation achieves very high performance. swap (CAS) instruction for all state changes, including the And it is sufficiently complete that we can measure normal “management” state changes, e.g., SMOs and flushes operation performance reliably and consistently. We exploit the database ability to cache updates until it is convenient to C. Flash Based, Log Structured post them to stable storage. This is a key to database system Early work with flash storage exploited in-page logging performance, and is explicitly enabled via control operations , placing log records near a B-tree page instead of updating in the Deuteronomy architecture’s interface between TC and the page “in-place” which would require an erase cycle. While DC. However, exploiting it, as we have done, means that one our delta records resemble somewhat in-page log records, they needs to be careful in comparing Bw-tree performance with have nothing to do with transactional recovery. We view them atomic record stores that immediately make update operations not as log records, but as the new versions of the records. stable.
12 .B. Log Structured Store fruit. While we were “confident” in our design choices, we We have focused this paper on the Bw-tree part of our were nonetheless pleasantly surprised by how good the results atomic record store (ARS) or data component (DC). Thus, we were. Supporting millions of operations per second on a single have not discussed issues related to managing LSS. A latch- “vanilla” cpu offers strong confirmation for our design choices. free environoment is very challenging for many elements of That we out-performed very good competing approaches by data management systems, including storage management as such a large margin was “icing on the cake”. done by our LSS. We will describe details of how the LSS is R EFERENCES implemented in another paper. Here we want to provide a few  “MongoDB. http://www.mongodb.org/.” highlights of areas needing attention.  J. J. Levandoski, D. B. Lomet, M. F. Mokbel, and K. Zhao, “Deuteron- • We manage our flush buffer in a latch-free way. 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