[MDEV-31309] The status variable Innodb_buffer_pool_read_requests is not updated correctly Created: 2023-05-19  Updated: 2023-12-17  Resolved: 2023-05-19

Status: Closed
Project: MariaDB Server
Component/s: Storage Engine - InnoDB
Affects Version/s: 10.11.3, 10.6.13, 10.8.8, 10.9.6, 10.10.4
Fix Version/s: 10.6.14, 10.9.7, 10.10.5, 10.11.4, 11.0.3, 11.1.2

Type: Bug Priority: Major
Reporter: Marko Mäkelä Assignee: Marko Mäkelä
Resolution: Fixed Votes: 0
Labels: regression

Issue Links:
Blocks
Problem/Incident
is caused by MDEV-26827 Make page flushing even faster Closed
Relates
relates to MDEV-32920 Innodb_buffer_pool_read_requests alwa... Closed

 Description   

In MDEV-26827, some monitoring was changed.
Some status variables were changed to point directly to the underlying variables, like this:

  {"buffer_pool_read_requests", &buf_pool.stat.n_page_gets, SHOW_SIZE_T},

Unlike all other variables in buf_pool.stat this particular one is divided into shards for performance reasons (MDEV-21212). So, the status variable would only reflect the contents of the first shard and not the entire counter. Thanks to dragle for noticing this.

Note: This counter is inaccurate by design. If we protected it correctly, it would introduce a performance bottleneck.

 Comments   
Comment by Daniel Ragle [ 2023-05-19 ]

Thanks, I believe this is taken care of in the 10.6.14 build you pointed me to in a separate thread. Unfortunately the only machine I had that displayed the problem was reverted early this week to 10.6.12, and when I try to load new 10.6.13 instances in VMs they don't display this behavior.

But not seeing the issue consistently would seem to make sense based on your explanation, and I'm not seeing it in the 10.6.14 build either, so this should be good.

Comment by Roger [ 2023-10-24 ]

This bug still appears to exist in 10.9.8.

MariaDB [(none)]> select count from information_schema.INNODB_METRICS where name = 'buffer_pool_read_requests';
-------

count

-------

83743

MariaDB [(none)]> show global status like 'Innodb_buffer_pool_read_requests';
---------------------------------------+

Variable_name Value

---------------------------------------+

Innodb_buffer_pool_read_requests 0

---------------------------------------+

Comment by Daniel Black [ 2023-12-17 ]

rogerd007 note the 10.9 series (and 10.10) are out of maintaince as they where short term support only.

For maintaince policy - see https://mariadb.org/about/#maintenance-policy

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