Checkpoints

checkpoint_segments

Parameter description: Specifies the minimum number of WAL segment files in the period specified by checkpoint_timeout. The size of each log file is 16 MB.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer. The minimum value is 1.

Increasing the value of this parameter speeds up the export of a large amount of data. Set this parameter based on checkpoint_timeout and shared_buffers. This parameter affects the number of WAL segment files that can be reused. Generally, the maximum number of reused files in the pg_xlog folder is twice the number of checkpoint_segments. The reused files are not deleted and are renamed to the WAL segment files which will be later used.

Default value: 64

checkpoint_timeout

Parameter description: Specifies the maximum time between automatic WAL checkpoints.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 30 to 3600. The unit is second.

If the value of checkpoint_segments is increased, you need to increase the value of this parameter. The increase of these two parameters further requires the increase of shared_buffers. Consider all these parameters during setting.

Default value: 15min

checkpoint_completion_target

Parameter description: Specifies the completion target of each checkpoint, as a fraction of total time between checkpoints.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: a double-precision floating point number ranging from 0.0 to 1.0

Default value: 0.5

NOTE: 0.5 indicates that each checkpoint should be complete within 50% of the interval between checkpoints.

checkpoint_warning

Parameter description: Specifies a time in seconds. If the checkpoint interval is close to this time due to filling of checkpoint segment files, a message is sent to the server log to suggest an increase in the checkpoint_segments value.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 0 to INT_MAX. The unit is second. 0 indicates that the warning is disabled.

Default value: 5min

Recommended value: 5min

checkpoint_wait_timeout

Parameter description: Sets the longest time that the checkpoint waits for the checkpointer thread to start.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 2 to 3600. The unit is second.

Default value: 1min

enable_incremental_checkpoint

Parameter description: Specifies whether to enable incremental checkpointing.

MogDB's default checkpoint is an incremental checkpoint. When a manual checkpoint is executed, it selects the LSN of the most recent WAL log entry as the checkpoint.redo point. It then waits for the pagewriter thread to perform dirty page flushing and complete fsync operations. After these operations are finished, MogDB updates the pg_control file.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

Default value: on

enable_double_write

Parameter description: Specifies whether to enable the doublewrite buffer. When the incremental checkpointing is enabled, enable_double_write is enabled at the same time and the doublewrite buffer instead of full_page_writes is used to prevent partial page writes.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

Default value: on

incremental_checkpoint_timeout

Parameter description: Specifies the maximum interval between automatic WAL checkpoints when the incremental checkpointing is enabled.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 1 to 3600. The unit is second.

Default value: 1min

enable_xlog_prune

Parameter description: Specifies whether the primary server reclaims logs when any standby server is disconnected.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• If this parameter is set to on, the primary node does not recycle logs when any standby node is disconnected.
• If this parameter is set to off, the primary node recycles logs when any standby node is disconnected.

Default value: on

max_redo_log_size

Parameter description: On the standby DN, this parameter specifies the maximum size of logs between the latest checkpoint and the current log replay location. On the primary DN, this parameter specifies the maximum size of logs between the recovery point and the latest log location. You are not advised to set this parameter to a large value if the RTO is concerned.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 163840 to 2147483647. The unit is kB.

Default value: 1GB

max_size_for_xlog_prune

Parameter description: This parameter takes effect when enable_xlog_prune is enabled. The working mechanism is as follows:

1. If all standby nodes specified by the replconninfo series GUC parameters are connected to the primary node, this parameter does not take effect.
2. If any standby node specified by the replconninfo series GUC parameters is not connected to the primary node, this parameter takes effect. When the number of historical logs on the primary node is greater than the value of this parameter, the logs are forcibly recycled. Exception: In synchronous commit mode (that is, the value of synchronous_commit is not local or off), if there are connected standby nodes, the primary node retains the logs that meet the minimum log receiving requirements on the majority of standby nodes. In this case, the number of reserved logs may exceed the value of max_size_for_xlog_prune.
3. If any standby node is being built, this parameter does not take effect. All logs of the primary node are retained to prevent build failures due to log recycling.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 0 to 2147483647. The unit is kB.

Default value: 2147483647