Problem Statement
What are the key differences between WiredTiger and MMAPv1 storage engines? Why was WiredTiger introduced?
Explanation
WiredTiger and MMAPv1 are two storage engines in MongoDB with significant differences in architecture and performance characteristics. MMAPv1 was the original storage engine but is now deprecated, while WiredTiger is the default since MongoDB 3.2.
The primary difference is concurrency control. MMAPv1 uses collection-level locking, meaning only one write operation can occur per collection at a time. This creates bottlenecks in write-heavy workloads. WiredTiger uses document-level locking, allowing multiple write operations to different documents in the same collection simultaneously. This dramatically improves write throughput.
Second, WiredTiger supports compression of both data and indexes using snappy or zlib algorithms. This reduces storage requirements by 50-80 percent and improves I/O performance because less data needs to be read from disk. MMAPv1 does not support compression.
Third, WiredTiger uses checkpoints and write-ahead logging for crash recovery and durability. Checkpoints are snapshots of data written to disk every 60 seconds, while the journal records operations between checkpoints. This provides better recovery guarantees than MMAPv1's journaling.
Fourth, WiredTiger uses a cache to hold frequently accessed data in memory. The cache size is configurable and defaults to 50 percent of RAM minus 1 GB. MMAPv1 relied on the operating system's file system cache, giving less control over memory usage.
WiredTiger was introduced to address MMAPv1's limitations in write performance, storage efficiency, and concurrency. It provides better performance for most workloads, especially write-heavy applications, and is now the only supported storage engine in modern MongoDB versions.
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