cfg_snapshot_daemon
Checkpoint daemon
The checkpoint daemon (snapshot daemon) is a constantly running
fiber. The checkpoint daemon creates a schedule for the
periodic snapshot creation based on the configuration options and the
speed of file size growth. If enabled, the daemon makes new
snapshot (.snap) files according to
this schedule.
The work of the checkpoint daemon is based on the following configuration options:
- checkpoint_interval – a new snapshot is taken once in a given period.
- checkpoint_wal_threshold – a new snapshot is taken once the size of all WAL files created since the last snapshot exceeds a given limit.
If necessary, the checkpoint daemon also activates the Tarantool garbage collector that deletes old snapshots and WAL files.
Tarantool garbage collector
Tarantool garbage collector can be activated by the checkpoint daemon. The garbage collector tracks the snapshots that are to be relayed to a replica or needed by other consumers. When the files are no longer needed, Tarantool garbage collector deletes them.
This garbage collector is called as follows:
- When the number of snapshots reaches the limit of checkpoint_count size. After a new snapshot is taken, Tarantool garbage collector deletes the oldest snapshot file and any associated WAL files.
- When the size of all WAL files created since the last snapshot reaches the limit of checkpoint_wal_threshold. Once this size is exceeded, the checkpoint daemon takes a snapshot, then the garbage collector deletes the old WAL files.
If an old snapshot file is deleted, the Tarantool garbage collector also deletes any write-ahead log (.xlog) files that meet the following conditions:
- The WAL files are older than the snapshot file.
- The WAL files contain information present in the snapshot file.
Tarantool garbage collector also deletes obsolete vinyl .run files.
Tarantool garbage collector doesn't delete a file in the following cases:
- A backup is running, and the file has not been backed up (see Hot backup).
- Replication is running, and the file has not been relayed to a replica (see Replication architecture),
- A replica is connecting.
- A replica has fallen behind. The progress of each replica is tracked; if a replica's position is far from being up to date, then the server stops to give it a chance to catch up. If an administrator concludes that a replica is permanently down, then the correct procedure is to restart the server, or (preferably) remove the replica from the cluster.
Since version 1.7.4.
The interval in seconds between actions by the
checkpoint daemon. If the option is set to a
value greater than zero, and there is activity that causes change to a
database, then the checkpoint daemon calls
box.snapshot() every checkpoint_interval seconds, creating a new snapshot
file each time. If the option is set to zero, the checkpoint daemon is
disabled.
Example
box.cfg{ checkpoint_interval = 7200 }
In the example, the checkpoint daemon creates a new database snapshot every two hours, if there is activity.
Type: integer
Default: 3600 (one hour)
Environment variable: TT_CHECKPOINT_INTERVAL
Dynamic: yes
Since version 1.7.4.
The maximum number of snapshots that are stored in the memtx_dir directory. If the number of snapshots after creating a new one exceeds this value, the Tarantool garbage collector deletes old snapshots. If the option is set to zero, the garbage collector does not delete old snapshots.
Example
box.cfg{checkpoint_interval = 7200,checkpoint_count = 3}
In the example, the checkpoint daemon creates a new snapshot every two hours until it has created three snapshots. After creating a new snapshot (the fourth one), the oldest snapshot and any associated write-ahead-log files are deleted.
Type: integer
Default: 2
Environment variable: TT_CHECKPOINT_COUNT
Dynamic: yes
Since version 2.1.2.
The threshold for the total size in bytes for all WAL files created since the last checkpoint. Once the configured threshold is exceeded, the WAL thread notifies the checkpoint daemon that it must make a new checkpoint and delete old WAL files.
This parameter enables administrators to handle a problem that could occur with calculating how much disk space to allocate for a partition containing WAL files.
Type: integer
Default: 10\^18 (a large number so in effect there is no limit by
default)
Environment variable: TT_CHECKPOINT_WAL_THRESHOLD
Dynamic: yes