What does a legacy MMAPv1 comparison teach you about tuning MongoDB WiredTiger?

Instruction: State the MMAPv1 removal boundary, explain WiredTiger concurrency, compression, and cache trade-offs, and choose workload measurements for evaluating performance.

Context: Assesses version-aware MongoDB storage knowledge and measurable performance decisions, while separating legacy migration from current engine configuration.

Updated

Example answer

MongoDB removed MMAPv1 in version 4.2. Comparing it with WiredTiger is useful for understanding a legacy migration, not choosing between engines for a current deployment. WiredTiger is MongoDB's default storage engine.

WiredTiger provides document-level write concurrency, snapshot-based reads, and compression. Its optimistic concurrency can encounter write conflicts and retry operations; document-level concurrency does not make contention disappear. Compression trades CPU for reduced disk space and I/O. Its internal cache and the operating system's filesystem cache both consume memory, and compressed disk size is not the in-memory working-set size.

For a current workload, measure query plans, index effectiveness, p95/p99 read and write latency, disk I/O, cache pressure, CPU, and replication lag under representative traffic. Compare supported compression choices and indexing changes using those measurements. Daily active users alone cannot identify a storage-engine bottleneck.

For an old MMAPv1 deployment, inspect its exact version, topology, and upgrade path. Follow the version-specific conversion procedure and verify a recoverable backup before migration. MMAPv1 data files cannot simply be opened as WiredTiger files. Validate document counts, indexes, application queries, and replication before advancing the upgrade.

Sources

WiredTiger behavior; MMAPv1 removal and conversion considerations.

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