Franck PachotDatabase Developer Advocate
All minibooks
Database field guide · 20
Pages, checkpoints, MVCC, and MongoDB persistence
A physical model of how MongoDB uses WiredTiger for B-tree storage, cache management, compression, checkpoints, and durable history.
Build the mental model before choosing the mechanism.
This minibook was AI-generated from Franck Pachot's archived blog posts. Links to the original articles are included for source context and verification.01
MongoDB maps collections and indexes to separate WiredTiger table URIs backed by .wt files. The catalog connects logical namespaces and idents to those physical objects.
02
WiredTiger reads and reconciles B-tree pages in its own cache. MongoDB's process memory therefore includes cache pages, engine metadata, sessions, and memory outside the configured cache.
03
When a dirty in-memory page is evicted or checkpointed, reconciliation writes a new on-disk representation. Updates can be split across pages and compressed before becoming blocks in the data file.
04
MongoDB supplies transaction timestamps while WiredTiger maintains update chains. Readers select a version appropriate to their read timestamp, and obsolete history can be removed only after it is no longer required.
05
Older committed values that no longer fit efficiently on the data page move to WiredTigerHS.wt. The history store lets readers reconstruct an older snapshot without keeping every version on the current page.
06
The journal makes committed changes recoverable between checkpoints, while checkpoints establish a consistent durable view of all data files. Startup recovery replays journal records newer than the checkpoint.
07
Concrete mechanics, diagnostic evidence, and executable patterns to carry into a real system.
MongoDB exposes collection and index idents in catalog metadata. Join those idents to WiredTiger table URIs before attributing file size or I/O to a namespace.
db.getSiblingDB('inventory').runCommand({ listCollections: 1 })
// Then inspect index names with db.collection.getIndexes();
// Correlate catalog idents with collection-*.wt and index-*.wt files.The useful WiredTiger cache signals distinguish bytes currently resident, dirty bytes, pages read and written, eviction work, and application threads forced to help eviction.
db.serverStatus().wiredTiger.cache
// Compare: bytes currently in the cache, tracked dirty bytes,
// pages read into cache, pages written from cache, and eviction counters.A pinned oldest timestamp prevents obsolete versions from being discarded and can grow the history store. Correlate transaction age, checkpoint progress, and history-store activity.
db.serverStatus().wiredTiger.transaction
db.currentOp({ active: true, secs_running: { $gte: 10 } })
// Investigate long snapshots before treating WiredTigerHS.wt growth as corruption.Use acknowledged write concern and a controlled crash test to verify which writes survive process and host failure. Filesystem copy tests while mongod is running do not replace supported backup or snapshot coordination.
db.events.insertOne(
{ requestId: UUID(), createdAt: new Date() },
{ writeConcern: { w: 'majority', j: true } }
)08
Optional deep dives with the complete experiments and product-version context behind this guide.