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SERVER-47293 Execution Architecture Guide: Checkpoints

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Gregory Wlodarek 2020-07-08 17:39:19 -04:00 committed by Evergreen Agent
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@ -628,6 +628,41 @@ explain how datafiles persist a machine’s identity which must be manipulated f
# Checkpoints
Checkpoints provide recovery points that enable the database to load a consistent snapshot of the
data quickly during startup or after a failure. Checkpoints provide basic operation durability in
favor of fast recovery in the event of a crash.
Write-ahead logging, aka [journaling](#journaling), is used in addition to checkpointing to provide
commit-level durability for all operations since the last checkpoint. On startup, all journaled
writes are re-applied to the data from the last checkpoint. Without journaling, all writes between
checkpoints would be lost.
Storage engines need to
[support checkpoints](https://github.com/mongodb/mongo/blob/r4.5.0/src/mongo/db/storage/storage_engine.h#L267)
for MongoDB to take advantage of this, otherwise MongoDB will act as an ephemeral data store. The
frequency of these checkpoints is determined by the
['storage.syncPeriodSecs' or 'syncdelay'](https://github.com/mongodb/mongo/blob/r4.5.0/src/mongo/db/mongod_options_storage.idl#L86-L93)
options.
The WiredTiger storage engine
[supports checkpoints](https://github.com/mongodb/mongo/blob/r4.5.0/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L443-L647)
, which are a read-only, static view of one or more data sources. When WiredTiger takes a
checkpoint, it writes all of the data in a snapshot to the disk in a consistent way across all of
the data files.
To avoid taking unnecessary checkpoints on an idle server, WiredTiger will only take checkpoints for
the following scenarios:
* When the [stable timestamp](../repl/README.md#replication-timestamp-glossary) is greater than or
equal to the [initial data timestamp](../repl/README.md#replication-timestamp-glossary), we take a
stable checkpoint, which is a durable view of the data at a particular timestamp. This is for
steady-state replication.
* The [initial data timestamp](../repl/README.md#replication-timestamp-glossary) is not set, so we
must take a full checkpoint. This is when there is no consistent view of the data, such as during
initial sync.
Not only does checkpointing provide us with durability for the database, but it also enables us to
take [backups of the data](#file-system-backups).
# Journaling
MongoDB provides write durability via a persisted change log for replicated writes and persistence