Background Writer

This section describes background writer parameters. The background writer process is used to write dirty data (new or modified data) in shared buffers to disks. This mechanism ensures that database processes seldom or never need to wait for a write action to occur when handling user queries.

It also mitigates performance deterioration caused by checkpoints because only a few of dirty pages need to be flushed to the disk when the checkpoints arrive. This mechanism, however, increases the overall net I/O load because while a repeatedly-dirtied page may otherwise be written only once per checkpoint interval, the background writer may write it several times as it is dirtied in the same interval. In most cases, continuous light loads are preferred, instead of periodical load peaks. The parameters discussed in this section can be set based on actual requirements.

bgwriter_thread_num

Parameter description: Specifies the number of bgwriter threads for flushing pages after the incremental checkpoint is enabled. Dirty pages to be evicted are flushed to disks, and non-dirty pages are placed in the candidate buffer chain. This parameter helps accelerate buffer eviction and improve performance.

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

Value range: an integer ranging from 0 to 8

• To test the effect of disabling this feature during development, you can set this parameter to 0. However, if this parameter is set to 0, the value will be changed to 1 in the code and the feature cannot be disabled.
• If this parameter is set to a value ranging from 1 to 8, the corresponding number of background threads are started to maintain the candidate buffer chain. Dirty pages that meet the conditions are flushed to disks, and non-dirty pages are added to the candidate list.

Default value: 2

bgwriter_delay

Parameter description: Specifies the interval at which the background writer writes dirty shared buffers. The background writer initiates write operations for some dirty shared buffers (the volume of data to be written is specified by the bgwriter_lru_maxpages parameter), sleep for the milliseconds specified by bgwriter_delay, and then restarts.

In many systems, the effective resolution of sleep delays is 10 milliseconds. Therefore, setting this parameter to a value that is not a multiple of 10 has the same effect as setting it to the next higher multiple of 10.

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

Value range: an integer ranging from 10 to 10000. The unit is millisecond.

Default value: 2s

Setting suggestion: Reduce this value in slow data writing scenarios to reduce the checkpoint load.

bgwriter_delay

Parameter description: Specifies the interval at which the background writer writes dirty shared buffers. The background writer initiates write operations for some dirty shared buffers (the volume of data to be written is specified by the bgwriter_lru_maxpages parameter), sleep for the milliseconds specified by bgwriter_delay, and then restarts.

In many systems, the effective resolution of sleep delays is 10 milliseconds. Therefore, setting this parameter to a value that is not a multiple of 10 has the same effect as setting it to the next higher multiple of 10.

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

Value range: an integer ranging from 10 to 10000. The unit is millisecond.

Default value: 2s

Setting suggestion: Reduce this value in slow data writing scenarios to reduce the checkpoint load.

candidate_buf_percent_target

Parameter description: Specifies the expected percentage of available buffers in the shared_buffer memory buffer in the candidate buffer chain when the incremental checkpoint is enabled. If the number of available buffers in the current candidate chain is less than the target value, the bgwriter thread starts flushing dirty pages that meet the requirements.

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 0.1 to 0.85

Default value: 0.3

bgwriter_lru_maxpages

Parameter description: Specifies the number of dirty buffers the background writer can write in each round.

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 1000

NOTE: When this parameter is set to 0, the background writer is disabled. This setting does not affect checkpoints.

Default value: 100

bgwriter_lru_multiplier

Parameter description: Specifies the coefficient used to estimate the number of dirty buffers the background writer can write in the next round.

The number of dirty buffers written in each round depends on the number of buffers used by server processes during recent rounds. The estimated number of buffers required in the next round is calculated using the following formula: Average number of recently used buffers x bgwriter_lru_multiplier. The background writer writes dirty buffers until sufficient clean and reusable buffers are available. The number of buffers the background writer writes in each round is always equal to or less than bgwriter_lru_maxpages.

Therefore, the value 1.0 represents a just-in-time policy of writing exactly the number of dirty buffers predicted to be required. Larger values provide some cushion against spikes in demand, whereas smaller values intentionally leave more writes to be done by server processes.

Smaller values of bgwriter_lru_maxpages and bgwriter_lru_multiplier reduce the extra I/O load caused by the background writer, but make it more likely that server processes will have to issue writes for themselves, delaying interactive queries.

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

Value range: a floating point number ranging from 0 to 10

Default value: 2

pagewriter_thread_num

Parameter description: Specifies the number of threads for background page flushing after the incremental checkpoint is enabled. Dirty pages are flushed in sequence to disks, promoting recovery points.

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

Value range: an integer ranging from 1 to 8

Default value: 2

dirty_page_percent_max

Parameter description: Specifies the percentage of dirty pages to shared_buffersafter the incremental checkpoint is enabled. When the value of this parameter is reached, the background page flush thread flushes dirty pages based on the maximum value of max_io_capacity.

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

Value range: a floating point number ranging from 0.1 to 1

Default value: 0.9

pagewriter_sleep

Parameter description: Specifies the interval for the pagewriter thread to flush dirty pages to disks after the incremental checkpoint is enabled. When the ratio of dirty pages to shared_buffers reaches dirty_page_percent_max, the number of pages in each batch is calculated based on the value of max_io_capacity. In other cases, the number of pages in each batch decreases proportionally.

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 3600000. The unit is ms.

Default value: 2000ms(2s)

max_io_capacity

Parameter description: Specifies the maximum I/O per second for the backend write process to flush pages in batches. Set this parameter based on the service scenario and disk I/O capability of the host. If the RTO is short or the data volume is much larger than the shared memory, and the service access data volume is random, the value of this parameter cannot be too small. A small parameter value reduces the number of pages flushed by the backend write process. If a large number of pages are eliminated due to service triggering, the services are affected.

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

Value range: an integer ranging from 30720 to 10485760. The unit is KB.

Default value: 512000 KB (500 MB)