Settings

wal_level

Parameter description: Specifies the level of information to be written to the WAL. The value cannot be empty or commented out.

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

• To enable WAL archiving and data streaming replication between primary and standby servers, set this parameter to archive, hot_standby, or logical.
• If this parameter is set to archive or minimal, hot_standby must be set to off. In a distributed environment, hot_standby cannot be set to off. Therefore, you are not advised to set this parameter to archive or minimal. Otherwise, the database cannot be started.

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 contain only basic information required for recovery from a database server crash or an emergency shutdown. Data cannot be restored from archived WALs.

• 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 are few measurable differences in performance between using hot_standby and archive levels. However, feedback is welcome if any differences in their impacts on product performance are noticeable.
• logical

  Only when this parameter is set to logical, logical logs can be parsed and the primary key information is recorded in Xlogs.

Default value: hot_standby

fsync

Parameter description: Specifies whether the GaussDB server uses the fsync() function (see wal_sync_method) to ensure that updates can be written to disks in a timely manner.

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

• Using the fsync() function ensures that the data can be recovered to a known state when an OS or a hardware crashes.
• Setting this parameter to off may result in unrecoverable data corruption in a system crash.

Value range: Boolean

• on indicates that the fsync() function is used.
• off indicates that the fsync() function is not used.

Default value: on

synchronous_commit

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

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: enumerated values

• on (true, yes, 1): The primary node waits for the standby node to flush logs to disks before committing a transaction.
• off (false, no, 0): The primary node commits a transaction without waiting for the primary node to flush logs to disks. This mode is also called asynchronous commit.
• local: The primary node waits for the primary node to flush logs to disks before committing a transaction. This mode is also called local commit.
• remote_write: The primary node waits for the standby node to write logs to the file system before committing a transaction. (The logs do not need to be flushed to disks.)
• remote_receive: The primary node waits for the standby node to receive logs before committing a transaction. (The logs do not need to be written to the file system.)
• remote_apply: The primary node waits for the standby node to complete log replay before committing a transaction.
• true: same as on.
• false: same as off.
• yes: same as on.
• no: same as off.
• 1: same as on.
• 0: same as off.
• 2: same as remote_apply.

Default value: on

wal_sync_method

Parameter description: Specifies the method used for forcing WAL updates out to disk.

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

If fsync is set to off, the setting of this parameter does not take effect because WAL file updates will not be forced out to disk.

Value range: enumerated values

• open_datasync indicates that WAL files are opened with the O_DSYNC option.
• fdatasync indicates that fdatasync() is called at each commit. (SLES 10 and SLES 11 are supported.)
• fsync_writethrough indicates that fsync() is called at each commit to force data in the buffer to be written to the disk.
  

  wal_sync_method can be set to fsync_writethrough on a Windows platform, but this setting has the same effect as setting the parameter to fsync on the Windows platform.

• fsync indicates that fsync() is called at each commit. (SLES 10 and SLES 11 are supported.)
• open_sync indicates that the open() with the O_SYNC option is used to write WAL files (SLES 10 and SLES 11 are supported).
  

  Not all platforms support the preceding parameters.

Default value: fdatasync

full_page_writes

Parameter description: Specifies whether the GaussDB server writes the entire content of each disk page to WALs during the first modification of that page after a checkpoint.

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

• This parameter is needed because a page write that is in process during an OS crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WALs will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored, but at the price of increasing the amount of data that must be written to WALs.
• Setting this parameter to off might lead to unrecoverable data corruption after a system failure. It might be safe to set this parameter to off if you have hardware (such as a battery-backed disk controller) or file-system software (such as ReiserFS 4) that reduces the risk of partial page writes to an acceptably low level.

Value range: Boolean

• on indicates that this feature is enabled.
• off indicates that this feature is disabled.

Default value: on

wal_log_hints

Parameter description: Specifies whether to write an entire page to WALs during the first modification of that page after a checkpoint, even for non-critical modifications of so-called hint bits. You are advised not to modify the setting.

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

Value range: Boolean

• on indicates that the entire page is written to WALs.
• off indicates that the entire page is not written to WALs.

Default value: on

wal_buffers

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

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

Value range: –1 to 2¹⁸. The minimum value is –1 and the maximum value is 262144. The unit is 8 KB.

• If this parameter is set to –1, the value of wal_buffers is automatically adjusted based on the value of shared_buffers. The default value is 1/32 of shared_buffers. If the value is less than 8, it will be forcibly set to 8. If the value is greater than 2048, it will be forcibly set to 2048.
• If this parameter is set to a value other than –1 and smaller than 4, the value 4 is forcibly used.

• Independent deployment: 1 GB (60-core CPU/480 GB memory and 32-core CPU/256 GB memory); 512 MB (16-core CPU/128 GB memory); 256 MB (8-core CPU/64 GB memory); 128 MB (4-core CPU/32 GB memory); 64 MB (4-core CPU/16 GB memory)

Setting suggestions: The content of the WAL buffer is written to disks at every transaction commit. Therefore, setting an extremely large value is unlikely to bring a significant increase in system performance. However, setting this parameter to hundreds of megabytes can improve the disk write performance on a server to which a large number of transactions are committed at the same time. The default value meets user requirements in most cases.

wal_writer_delay

Parameter description: Specifies the delay between activity rounds for the WAL writer.

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

A longer delay might lead to insufficient WAL buffer and a shorter delay leads to continuously writing of the WALs, thereby increasing the load of disk I/O.

Value range: an integer ranging from 1 to 10000. The unit is millisecond.

Default value: 200 ms

commit_delay

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

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

• When this parameter is set to a non-zero value, the committed transaction is stored in the WAL buffer instead of being written to the WAL immediately. Then the WAL writer 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 other transactions are ready to be committed, the delay is a waste of time.

Value range: an integer ranging from 0 to 100000. The unit is microsecond. 0 indicates no delay.

Default value: 0

commit_siblings

Parameter description: Specifies a threshold on the number of concurrent open transactions. If the number of concurrent open transactions is greater than the value of this parameter, a transaction that initiates a commit request will wait for a period of time specified by commit_delay. Otherwise, this transaction is written into WALs immediately.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 0 to 1000.

Default value: 5

wal_block_size

Parameter description: Specifies the size of a WAL disk block.

This is a fixed INTERNAL parameter. It can be viewed but cannot be modified.

Value range: an integer. The unit is byte.

Default value: 8192

wal_segment_size

Parameter description: Specifies the size of a WAL segment file.

This is a fixed INTERNAL parameter. It can be viewed but cannot be modified.

Value range: an integer. The unit is 8 KB.

Default value: 16 MB (2048 x 8 KB)

force_promote

Parameter description: Specifies whether to enable the forcible switchover function on the standby node.

When a cluster is faulty, the forcible switchover enables the cluster to recover services as soon as possible at the cost of losing some data. This is an escape method used when the cluster is unavailable. You are not advised to trigger this method frequently. You are not advised not to use this function if you are not clear about the impact of data loss on services.

To use this function, you need to enable it on the DN and CM Server and restart the cluster for the setting to take effect. For details about how to enable the forcible switchover function on the standby node, see "Emergency Handling > Performing a Forcible Primary/Standby Switchover" in Troubleshooting.

Value range: an integer. The value can be 0 (disabled) or 1 (enabled).

Default value: 0

wal_file_init_num

Parameter description: Specifies the number of Xlog segment files created by the WAL writer assistant thread at a time.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 2.

Value range: an integer ranging from 0 to 1000000.

Default value: 10

wal_debug

Parameter description: Specifies whether to output WAL-related debugging information. This parameter is available only when WAL_DEBUG is enabled during compilation.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

Default value: false

walwriter_cpu_bind

Parameter description: Sets the number of CPU cores bound to the WAL writer thread.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 2.

Value range: an integer ranging from –1 to 2147483647

Default value: –1

walwriter_sleep_threshold

Parameter description: Specifies the number of times that the idle Xlog is refreshed before the Xlog refresher enters sleep.

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

Value range: an integer ranging from 1 to 50000.

Default value: 500

wal_flush_timeout

Parameter description: Specifies the timeout interval for traversing WalInsertStatusEntryTbl. It is the maximum wait time for the adaptive Xlog disk flushing I/O to traverse WalInsertStatusEntryTbl.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 2.

If the timeout interval is too long, the Xlog flushing frequency may decrease, reducing the Xlog processing performance.

Value range: an integer ranging from 0 to 90000000. The unit is microsecond.

Default value: 2us

wal_flush_delay

Parameter description: Specifies the wait interval when an entry in the WAL_NOT_COPIED state is encountered during WalInsertStatusEntryTbl traversal.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 2.

Value range: an integer ranging from 0 to 90000000. The unit is microsecond.

Default value: 1us