高autovacuum_vacuum_cost_limit -为什么？

Question

更新2022-11-10:我已经为这件事开了一个AWS的案子，一旦他们回复，我会在这里告诉你。。

Postgres 12.9 AWS是在db.r5.4xlarge上管理的，它有64 on内存。

autovacuum_vacuum_cost_limit为1800：

select setting from pg_settings where name = 'autovacuum_vacuum_cost_limit'; -- 1800

AWS控制台中的参数组：

autovacuum_vacuum_cost_limit: GREATEST({log(DBInstanceClassMemory/21474836480)*600},200)

rds.adaptive_autovacuum: 1

autovacuum_vacuum_cost_limit IMHO的计算：

64 Gigabytes = 68,719,476,736 Bytes

GREATEST({log(68719476736/21474836480)*600},200)

GREATEST({log(3.2)*600},200)

GREATEST({0.50514997832*600},200)

GREATEST(303.089986992,200)

CloudWatch米制MaximumUsedTransactionIDs徘徊在200米左右。许多桌子接近2亿元。

所以autovacuum_vacuum_cost_limit 应该是303IMHO？为什么autovacuum_vacuum_cost_limit 在1800点？我想是303。我想我可以排除人工干预。谢谢，

Answer 1

这是aws的答案。简言之，这一解释有两个方面：

1. 指的是"log“--它们指的是"log base 2”而不是"log base 10“
2. --它们在

的某一点上四舍五入

The autovacuum_vacuum_cost_limit formula is GREATEST({log(DBInstanceClassMemory/21474836480)*600},200).

Log in above formula is log of base 2 and the log value is rounded off before it is multiplied by 600.

As your instance's instance type is r5.2xlarge, the instance class memory is 64 gb.

DBInstanceClassMemory= 64 GiB =68719476736 bytes

Therefore, the following calculation is used to calculate the autovacuum_vacuum_cost_limit value:

GREATEST({log(68719476736/21474836480)600},200)
= GREATEST({log(3.2)600},200) --> log base 2 of (3.2) is 1.678 which is rounded off to 2
= GREATEST({2*600},200)
= GREATEST({1200},200)
= 1200