The amounts of data processed by applications are constantly growing. With this growth, scaling storage becomes more challenging. Every database system has its own trade-offs. Understanding them is crucial as it helps in selecting the right one from so many available choices.
Every application is different in terms of read/write workload balance, consistency requirements, latencies, and access patterns. Familiarizing yourself with database and storage internals facilitates architectural decisions, helps explain why a system behaves a certain way, helps troubleshoot problems when they arise, and fine-tunes the database for your workload.
It is impossible to optimize a system in all directions. In an ideal world there would be data structures guaranteeing the best read and write performance with no storage overhead but, of course, in practice that is not possible.
This article takes a closer look at two storage system design approaches used in a majority of modern databases—read-optimized B-trees3 and write-optimized LSM (log-structured merge)-trees8—and describes their use cases and trade-offs.
B-trees are a popular read-optimized indexing data structure and generalization of binary trees. They come in many variations and are used in many databases (including MySQL InnoDB7 and PostgreSQL10) and even file systems (HFS+,1 HTrees in ext46). The B in B-tree stands for Bayer, the author of the original data structure, or Boeing, where he worked at that time.
In a binary tree every node has two children (referred as a left and a right child). Left and right subtrees hold the keys that are less than and greater than the current node key, respectively. To keep the tree depth to a minimum, a binary tree has to be balanced: when randomly ordered keys are being added to the tree, it is natural that one side of the tree will eventually get deeper than the other.
One way to rebalance a binary tree is to use so-called rotation: rearrange nodes, pushing the parent node of the longer subtree down below its child and pulling this child up, effectively placing it in its parent's position. Figure 1 is an example of rotation used for balancing in a binary tree. On the left, a binary tree is unbalanced after adding node 2 to it. In order to balance the tree, node 3 is used as a pivot (the tree is rotated around it). Then node 5, previously a root node and a parent node for 3, becomes its child node. After the rotation step is done, the height of the left subtree decreases by one and the height of the right subtree increases by one. The maximum depth of the tree has decreased.
Binary trees are most useful as in-memory data structures. Because of balancing (the need to keep the depth of all subtrees to a minimum) and low fanout (a maximum of two pointers per node), they do not work well on disk. B-trees allow for storing more than two pointers per node and work well with block devices by matching the node size to the page size (for example, 4KB). Some implementations today use larger node sizes, spanning across multiple pages in size.
B-trees have the following properties:
This article discusses the B+tree,5 a modern variant of the B-tree often used for database storage. The B+tree is different from the original B-tree3 in that it has an additional level of linked leaf nodes holding the values, and these values cannot be stored on internal nodes.
Anatomy of the B-tree. Let's first take a closer look at the B-tree building blocks, illustrated in Figure 2. B-trees have several node types: root, internal, and leaf. Root (top) is the node that has no parents (that is, it is not a child of any other node). Internal nodes (middle) have both a parent and children; they connect a root node with leaf nodes. Leaf nodes (bottom) carry the data and have no children. Figure 2 depicts a B-tree with a branching factor of four (four pointers, three keys in internal nodes, and four key/value pairs on leaves).
B-trees are characterized by the following:
Every non leaf node in the tree holds up to N-1 keys (index entries), separating the tree into N subtrees that can be located by following a corresponding pointer. Pointer i from an entry Ki points to a subtree in which all index entries are such that Ki-1 <= Ksearched > Ki (where K is a set of keys). The first and last pointers are special cases, pointing to subtrees in which all the entries are less than or equal to K0 in the case of the leftmost child, or greater than KN in the case of the rightmost child. A leaf node may also hold a pointer to the previous and next nodes on the same level, forming a doubly linked list of sibling nodes. Keys in all the nodes are always sorted.
Lookups. When performing lookups, the search starts at the root node and follows internal nodes recursively down to the leaf level. On each level, the search space is reduced to the child subtree (the range of this subtree includes the searched value) by following the child pointer. Figure 3 shows a lookup in a B-tree making a single root-to-leaf pass, following the pointers "between" the two keys, one of which is greater than (or equal to) the searched term, and the other of which is less than the searched term. When a point query is performed, the search is complete after locating the leaf node. During the range scan, the keys and values of the found leaf, and then the sibling leaf's nodes, are traversed, until the end of the range is reached.
In terms of complexity, B-trees guarantee log(n) lookup, because finding a key within the node is performed using binary search, shown in Figure 4. Binary search is easily explained in terms of searching for words beginning with a certain letter in the dictionary, where all words are sorted alphabetically. First you open the dictionary exactly in the middle. If the searched letter is alphabetically "less than" (appears earlier than) the one opened, you continue your search in the left half of the dictionary; otherwise, you continue in the right half. You keep reducing the remaining page range by half and picking the side to follow until you find the desired letter. Every step halves the search space, making the lookup time logarithmic. Searches in B-trees have logarithmic complexity, since on the node level keys are sorted, and the binary search is performed in order to find a match. This is also why it is important to keep the occupancy high and uniform across the tree.
Insertions, updates, and deletions. When performing insertions, the first step is to locate the target leaf. For that, the aforementioned search algorithm is used. After the target leaf is located, key and value are appended to it. If the leaf does not have enough free space, this situation is called overflow, and the leaf has to be split in two. This is done by allocating a new leaf, moving half the elements to it and appending a pointer to this newly allocated leaf to the parent. If the parent does not have free space either, a split is performed on the parent level as well. The operation continues until the root is reached. When the root overflows, its contents are split between the newly allocated nodes, and the root node itself is overwritten in order to avoid relocation. This also implies the tree (and its height) always grows by splitting the root node.
LSM-trees. The log-structured merge-tree is an immutable disk-resident write-optimized data structure. It is most useful in systems where writes are more frequent than lookups that retrieve the records. LSM-trees have been getting more attention because they can eliminate random insertions, updates, and deletions.
Anatomy of the LSM-tree. To allow sequential writes, LSM-trees batch writes and updates in a memory-resident table (often implemented using a data structure allowing logarithmic time lookups, such as a binary search tree or skip list) until its size reaches a threshold, at which point it is written on disk (this operation is called a flush). Retrieving the data requires searching all disk-resident parts of the tree, checking the in-memory table, and merging their contents before returning the result. Figure 5 shows the structure of an LSM-tree: a memory-resident table used for writes, disk-resident SSTables are used for reads. Whenever the memory table is large enough, its sorted contents are written on disk. Reads are served, hitting both disk- and memory-resident tables, requiring a merge process to reconcile the data.
Sorted string tables. Many modern LSM-tree implementations (such as RocksDB and Apache Cassandra) implement disk-resident tables as Sorted String Tables (SSTables), because of their simplicity (easy to write, search, and read) and merge properties (during the merge, source SSTable scans and merged result writes are sequential).
An SSTable is a disk-resident ordered immutable data structure. Structurally, an SSTable is split into two parts: data and index blocks, as shown in Figure 6. A data block consists of sequentially written unique key/value pairs, ordered by key. An index block contains keys mapped to data-block pointers, pointing to where the actual record is located. An index is often implemented using a format optimized for quick searches, such as a B-tree, or using a hash table for a point-query. Every value item in an SSTable has a time-stamp associated with it. This specifies the write time for inserts and updates (which are often indistinguishable) and removal time for deletes.
SSTables have some nice properties:
An SSTable represents a snapshot of all database operations over a period of time, as the SSTable is created by the flush process from the memory-resident table that served as a buffer for operations against the database state for this period.
Lookups. Retrieving data requires searching all SSTables on disk, checking the memory-resident tables, and merging their contents before returning the result. The merge step during the read is required since the searched data can reside in multiple SSTables.
The merge step is also necessary to ensure the deletes and updates work. Deletes in an LSM-tree insert placeholders (often called tombstones), specifying which key was marked for deletion. Similarly, an update is just a record with a later timestamp. During the read, the records that get shadowed by deletes are skipped and not returned to the client. A similar thing happens with the updates: out of two records with the same key, only the one with the later timestamp is returned. Figure 7 shows a merge step reconciling the data stored in separate tables for the same key: as shown here, the record for Alex was written with timestamp 100 and updated with a new phone and timestamp 200; the record for John was deleted. The other two entries are taken as is, as they are not shadowed.
To reduce the number of searched SSTables and to avoid checking every SSTable for the searched key, many storage systems employ a data structure known as a Bloom Filter.2 This is a probabilistic data structure that can be used to test whether an element is a member of the set. It can produce false-positive matches (that is, state that the element is a member of set, while it is not, in fact, present there) but cannot produce false negatives (that is, if a negative match is returned, the element is guaranteed not to be a member of the set). In other words, a Bloom Filter is used to tell if the key "might be in an SSTable" or "is definitely not in an SSTable." SSTables for which a Bloom Filter has returned a negative match are skipped during the query.
LSM maintenance. Since SSTables are immutable, they are written sequentially and hold no reserved empty space for in-place modifications. This means insert, update, or delete operations would require rewriting the whole file. All operations modifying the database state are "batched" in the memory-resident table. Over time, the number of disk-resident tables will grow (data for the same key located in several files, multiple versions of the same record, redundant records that got shadowed by deletes), and the reads will continue getting more expensive.
To reduce the cost of reads, reconcile space occupied by shadowed records, and reduce the number of disk-resident tables, LSM-trees require a compaction process that reads complete SSTables from disk and merges them. Because SSTables are sorted by key and compaction works like merge-sort, this operation is very efficient: records are read from several sources sequentially, and merged output can be appended to the results file right away, also sequentially. One of the advantages of merge-sort is that it can work efficiently even for merging large files that don not fit in memory. The resulting table preserves the order of the original SSTables.
During this process, merged SSTables are discarded and replaced with their "compacted" versions, as shown in Figure 8. Compaction takes multiple SSTables and merges them into one. Some database systems logically group the tables of the same size to the same "level" and start the merge process whenever enough tables are on a particular level. After compaction, the number of SSTables that have to be addressed is reduced, making queries more efficient.
To reduce the number of I/O operations and make them sequential, both B-trees and LSM-trees batch operations in memory before making an actual update. This means data integrity is not guaranteed during failure scenarios and atomicity (applying a series of changes atomically, as if they were a single operation, or not applying them at all) and durability (ensuring that in the face of a process crash or power loss, data has reached persistent storage) properties are not ensured.
To solve that problem, most modern storage systems employ WAL (write-ahead logging). The key idea behind WAL is that all the database state modifications are first durably persisted in the append-only log on disk. If the process crashes in the middle of an operation, the log is replayed, ensuring no data is lost and all changes appear atomically.
One of the biggest differences between B-tree and LSM-tree data structures is what they optimize for and what implications these optimizations have.
In B-trees, using WAL can be understood as writing changes to data files only after they have been logged. Usually log sizes for B-tree storage systems are relatively small: as soon as changes are applied to the persisted storage, they can be discarded. WAL serves as a backup for the in-flight operations: any changes that were not applied to data pages can be redone from the log records.
In LSM-trees, WAL is used to persist changes that have reached the memtables but have not yet been fully flushed on disk. As soon as a memtable is fully flushed and switched so that read operations can be served from the newly created SSTable, the WAL segment holding the data for the flushed memtable can be discarded.
One of the biggest differences between the B-tree and LSM-tree data structures is what they optimize for and what implications these optimizations have.
Let's compare the properties of B-trees with LSM-trees. In summary, B-trees have the following properties:
LSM-trees have these properties:
Developing storage systems always presents the same challenges and factors to consider. Deciding what to optimize for has a substantial influence on the result. You can spend more time during write in order to lay out structures for more efficient reads, reserve extra space for in-place updates, facilitate faster writes, and buffer data in memory to ensure sequential write operations. It is impossible, however, to do this all at once. An ideal storage system would have the lowest read cost, lowest write cost, and no overhead. In practice, data structures compromise among multiple factors. Understanding these compromises is important.
Researchers from Harvard's DASlab (Data System Laboratory) summarized the three key parameters database systems are optimized for: read overhead, update overhead, and memory overhead, or RUM. Understanding which of these parameters are most important for your use-case influences the choice of data structures, access methods, and even suitability for certain workloads, as the algorithms are tailored having a specific use-case in mind.
The RUM Conjecture4 states that setting an upper bound for two of the mentioned overheads also sets a lower bound for the third one. For example, B-trees are read-optimized at the cost of write overhead as well as having to reserve empty space for the (thereby resulting in memory overhead). LSM-trees have less space overhead at a cost of read overhead brought on by having to access multiple disk-resident tables during the read. These three parameters form a competing triangle, and improvement on one side may imply compromise on the other. Figure 9 illustrates the RUM Conjecture.
B-trees optimize for read performance: the index is laid out in a way that minimizes the disk accesses required to traverse the tree. Only a single index file has to be accessed to locate the data. This is achieved by keeping this index file mutable, which also increases write amplification resulting from node splits and merges, relocation, and fragmentation/imbalance-related maintenance. To amortize update costs and reduce the number of splits, B-trees reserve extra free space in nodes on all levels. This helps to postpone write amplification until the node is full. In short, B-trees trade update and memory overhead for better read performance.
LSM-trees optimize for write performance. Neither updates nor deletes require locating data on disk (which B-trees do), and they guarantee sequential writes by buffering all insert, update, and delete operations in memory-resident tables. This comes at the price of higher maintenance costs and a need for compaction (which is just a way of mitigating the ever-growing price of reads and reducing the number of disk-resident tables) and more expensive reads (as the data has to be read from multiple sources and merged). At the same time, LSM-trees eliminate memory overhead by not reserving any empty space (unlike B-tree nodes, which have an average occupancy of 70%, an overhead required for in-place updates) and allowing block compression because of the better occupancy and immutability of the end file. In short, LSM-trees trade read performance and maintenance for better write performance and lower memory overhead.
There are data structures that optimize for each desired property. Using adaptive data structures allows for better read performance at the price of higher maintenance costs. Adding metadata facilitating traversals (such as fractional cascading) will have an impact on write time and take space, but can improve the read time. Optimizing for memory efficiency using compression (for example, algorithms such as Gorilla compression,9 delta encoding, and many others) will add some overhead for packing the data on writes and unpacking it on reads. Sometimes, you can trade functionality for efficiency. For example, heap files and hash indexes can provide great performance guarantees and smaller space overhead because of the file format simplicity, for the price of not being able to perform anything but point queries. You can also trade precision for space and efficiency by using approximate data structures, such as the Bloom Filter, HyperLogLog, Count-Min sketch, and many others.
The three tunables—read, update, and memory overheads—can help you evaluate the database and gain a deeper understanding of the workloads for which it is best suited. All of them are quite intuitive, and it is often easy to sort the storage system into one of the buckets and guess how it is going to perform, then validate your hypothesis through extensive testing.
Of course, there are other important factors to consider when evaluating a storage system, such as maintenance overhead, operational simplicity, system requirements, suitability for frequent updates and deletes, access patterns, and so on. The RUM Conjecture is just a rule of thumb that helps to develop an intuition and provide an initial direction. Understanding your workload is the first step on the way to building a scalable back end.
Some factors may vary from implementation to implementation, and even two databases that use similar storage-design principles may end up performing differently. Databases are complex systems with many moving parts and are an important and integral part of many applications. This information will help you peek under the hood of a database and, knowing the difference between the underlying data structures and their inner doings, decide what is best for you.
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