Settings

wal_level

Parameter description: Specifies the level of the information that is written to WALs.

Type: POSTMASTER

Value range: enumerated values

• minimal

  Advantages: Certain bulk operations (including creating tables and indexes, executing cluster operations, and copying tables) are safely skipped in logging, which can make those operations much faster.

  Disadvantages: WALs only contain basic information required for the recovery from a database server crash or an emergency shutdown. Archived WALs cannot be used to restore data.

• archive

  Adds logging required for WAL archiving, supporting the database restoration from archives.

• hot_standby
  • Further adds information required to run SQL queries on a standby server and takes effect after a server restart.
  • To enable read-only queries on a standby server, the wal_level parameter must be set to hot_standby on the primary server and the same value must be set on the standby server. There is little measurable difference in performance between using hot_standby and archive levels, so feedback is welcome if any production performance impacts are noticeable.

Default value: hot_standby

• To enable WAL archiving and data streaming replication between primary and standby servers, set this parameter to archive or hot_standby.
• If this parameter is set to archive, hot_standby must be set to off. Otherwise, the database startup fails.

synchronous_commit

Parameter description: Specifies the synchronization mode of the current transaction.

Type: USERSET

Value range: enumerated values

• on indicates synchronization logs of a standby server are flushed to disks.
• off indicates asynchronous commit.
• local indicates local commit.
• remote_write indicates synchronization logs of a standby server are written to disks.
• remote_receive indicates synchronization logs of a standby server are required to receive data.

Default value: on

wal_buffers

Parameter description: Specifies the number of XLOG_BLCKSZs used for storing WAL data. The size of each XLOG_BLCKSZ is 8 KB.

Type: POSTMASTER

Value range: -1 to 2¹⁸. The unit is 8 KB.

• If this parameter is set to -1, the value of wal_buffers is automatically changed to 1/32 of shared_buffers. The minimum value is 8 x XLOG_BLCKSZ, and the maximum value is 2048 x XLOG_BLCKSZ.
• If it is set to a value smaller than 8, the value 8 is used. If it is set to a value greater than 2048, the value 2048 is used.

Default value: 16 MB

Setting suggestions: The content of WAL buffers is written to disks at each transaction commit, and setting this parameter to a large value does not significantly improve system performance. Setting this parameter to hundreds of megabytes can improve the disk writing performance on the server, to which a large number of transactions are committed. Based on experiences, the default value meets user requirements in most cases.

commit_delay

Parameter description: Specifies the duration of committed data be stored in the WAL buffer.

Type: USERSET

Value range: an integer, ranging from 0 to 100000 (unit: μs). 0 indicates no delay.

Default value: 0

• When this parameter is set to a value other than 0, the committed transaction is stored in the WAL buffer instead of being written to the WAL immediately. Then, the WALwriter process flushes the buffer out to disks periodically.
• If system load is high, other transactions are probably ready to be committed within the delay. If no transactions are waiting to be submitted, the delay is a waste of time.

commit_siblings

Parameter description: Specifies a limit on the number of ongoing transactions. If the number of ongoing transactions is greater than the limit, a new transaction will wait for the period of time specified by commit_delay before it is submitted. If the number of ongoing transactions is less than the limit, the new transaction is immediately written into a WAL.

Type: USERSET

Value range: an integer ranging from 0 to 1000

Default value: 5

enable_xlog_group_insert

Parameter description: Specifies whether to enable the group insertion mode for WALs. Only the Kunpeng architecture supports this parameter.

Type: SIGHUP

Value range: Boolean

• on: enabled
• off: disabled

Default value: on

wal_compression

Parameter description: Specifies whether to compress FPI pages.

Type: USERSET

Value range: Boolean

• on: enable the compression
• off: disable the compression

Default value: on

• Only zlib compression algorithm is supported.
• For clusters that are upgraded to the current version from an earlier version, this parameter is set to off by default. You can run the gs_guc command to enable the FPI compression function if needed.
• If the current version is a newly installed version, this parameter is set to on by default.
• If this parameter is manually enabled for a cluster upgraded from an earlier version, the cluster cannot be rolled back.

wal_compression_level

Parameter description: Specifies the compression level of zlib compression algorithm when the wal_compression parameter is enabled.

Type: USERSET

Value range: an integer ranging from 0 to 9.

• 0 indicates no compression.
• 1 indicates the lowest compression ratio.
• 9 indicates the highest compression ratio.

Default value: 9