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Cost-based Vacuum Delay

Updated at: Mar 13, 2020 GMT+08:00

This feature allows administrators to reduce the I/O impact of the VACUUM and ANALYZE statements on concurrent database activities. It is often more important to prevent maintenance statements, such as VACUUM and ANALYZE, from affecting other database operations than to run them quickly. Cost-based vacuum delay provides a way for administrators to achieve this purpose.

Certain operations hold critical locks and should be complete as quickly as possible. In DWS, cost-based vacuum delays do not take effect during such operations. To avoid uselessly long delays in such cases, the actual delay is calculated as follows and is the maximum value of the following calculation results:

  • vacuum_cost_delay*accumulated_balance/vacuum_cost_limit
  • vacuum_cost_delay*4

Context

During the execution of the ANALYZE | ANALYSE and VACUUM statements, the system maintains an internal counter that keeps track of the estimated cost of the various I/O operations that are performed. When the accumulated cost reaches a limit (specified by vacuum_cost_limit), the process performing the operation will sleep for a short period of time (specified by vacuum_cost_delay). Then, the counter resets and the operation continues.

By default, this feature is disabled. To enable this feature, set vacuum_cost_delay to a value other than 0.

vacuum_cost_delay

Parameter description: Specifies the length of time that the process will sleep when vacuum_cost_limit has been exceeded.

On many systems, the effective resolution of sleep length is 10 ms. 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 set to a small value, such as 10 or 20 milliseconds. Adjusting vacuum's resource consumption is best done by changing other parameters.

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

Value range: an integer (unit: ms). A positive number enables cost-based vacuum delay and 0 disables cost-based vacuum delay.

Default value: 0

vacuum_cost_page_hit

Parameter description: Specifies the estimated cost for vacuuming a buffer found in the shared buffer. It represents the cost to lock the buffer pool, look up the shared Hash table, and scan the page.

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

Value range: integer (ms)

Default value: 1

vacuum_cost_page_miss

Parameter description: Specifies the estimated cost for vacuuming a buffer read from the disk. It represents the cost to lock the buffer pool, look up the shared Hash table, read the desired block from the disk, and scan the block.

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

Value range: integer (ms)

Default value: 10

vacuum_cost_page_dirty

Parameter description: Specifies the estimated cost charged when vacuum modifies a block that was previously clean. It represents the I/Os required to flush the dirty block out to disk again.

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

Value range: integer (ms)

Default value: 20

vacuum_cost_limit

Parameter description: Specifies the cost limit. The cleanup process will sleep if this limit is exceeded.

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

Value range: integer (ms)

Default value: 200

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