LSM databases — An Overview: The DB for write heavy workloads
Log-Structured Merge-tree (LSM) databases are a type of database that organizes data for efficient insertion, updates, and deletion operations, making them particularly well-suited for write-heavy workloads.
Popular LSM databases include Apache Cassandra, , LevelDB, and ScyllaDB. Each database has unique features and optimizations, but they all leverage the underlying principles of LSM data structures to provide efficient performance for write-heavy workloads. LSM principles are also used in Kafka and HBase
Overview of LSM databases:
1. Multi layer Data Organization for Fast write :
In LSM databases, data is organized into multiple layers. These typically include a write-ahead log (WAL) for sequential writes, an in-memory data structure called a memtable for recent writes, and one or more on-disk data structures for long-term storage.
Main Components:
- Write-ahead Log (WAL): Sequentially records all write operations to ensure durability and maintain write order.
- Memtable: In-memory data structure that caches recent write operations for fast access.
- Sorted String Table (SSTable): On-disk data structure containing sorted key-value pairs, usually generated from flushed memtables or compacted SSTables.
2. Write Optimization strategies:
LSM databases optimize write operations by
- First incoming data is written sequentially to a WAL. This ensures durability and maintains write order.
- Then, the data is added to an in-memory memtable for fast writes.
- Periodically, the memtable is flushed to disk, usually in sorted order, creating a new on-disk data structure known as a sorted string table (SSTable).
3. Periodic Merge Operations for efficient storage:
As SSTables accumulate on disk, LSM databases periodically merge them together to maintain efficient storage and query performance.
- During a merge operation, overlapping SSTables are combined and compacted to produce a new, larger SSTable with sorted and deduplicated data.
- This process helps reduce disk space usage and improves query performance by reducing the number of disk seeks required for read operations.
4. Sorted data for efficient Query Processing:
LSM databases support efficient query processing by maintaining sorted data on disk.
- When querying data, LSM databases typically search through the in-memory memtable and the on-disk SSTables in a sorted manner, leveraging techniques like binary search to efficiently locate and retrieve data.
5. Configurable Consistency and Durability:
LSM databases typically provide configurable levels of consistency and durability.
- Write operations are durable due to the use of a WAL, and durability guarantees can be adjusted based on application requirements.
Real World Use Case:
- LSM databases are commonly used in various applications, including time-series databases, distributed systems, and Big Data analytics platforms.
- The append-only nature of write operations and the ability to batch writes and merge data efficiently make LSM databases well-suited for high-throughput environments.
Drawbacks of LSM Databases:
- Relatively higher read latency: LSM databases may suffer from performance during read-heavy workloads. This is because data may be spread across multiple SSTables, requiring additional disk seeks to locate and retrieve the desired data.
- Write process overhead: Although LSM databases offer high write throughput, individual write operations may experience higher latency compared to traditional B-tree-based databases. This is due to the additional overhead of write-ahead logging, memtable flushes, and compaction processes.
Alternatives to LSM:
- B-tree-based Databases: B-tree-based databases use balanced tree structures which provide efficient read and write operations without the need for compaction processes. Used in : PostgreSQL, MySQL.
- It provides more balanced performance for sequential and random access. It is efficient for range queries as traversing the tree can quickly identify a range of keys.
2. In-memory Databases: In-memory databases store data entirely in RAM, offering extremely low-latency read and write operations.
- They have limited storage capacity, but can provide excellent performance for certain use case like cache. Example : Redis and Memcached.
3. LSM-Tree Hybrid Databases: Some databases combine LSM and B-tree structures to leverage the strengths of both approaches.
- Example : WiredTiger (used in MongoDB)
