Bug #35030 maria_pagecache_read* status variables updated at wrong moment

• Submitted: 4 Mar 2008 10:23
• Modified: 16 Mar 2008 16:08
• Reporter: Guilhem Bichot
• Status: Closed
• Category: MySQL Server: Maria storage engine
• Severity: S3 (Non-critical)
• Version: 5.1-maria
• OS: Any
• Assigned to: Oleksandr Byelkin
• CPU Architecture: Any

[4 Mar 2008 10:23] Guilhem Bichot

Description:
First, here's what the manual says:

  Key_read_requests:
  The number of requests to read a key block from the cache.
  Key_reads:
  The number of physical reads of a key block from disk. If Key_reads is
  large, then your key_buffer_size value is probably too small. The
  cache miss rate can be calculated as Key_reads/Key_read_requests.

So, Key_reads is about physical reads.

In key cache, it is incremented at places where we do a pread(), that
looks logical.
In page cache, it's incremented in find_block(), when taking a block
from the LRU. That is strange: pagecache_unlock(), pagecache_unpin(),
pagecache_write_part() all call find_block(). There is no reason that
a pagecache_write_part() should increase global_cache_read.
And also, find_block() increments the counter even for secondary read
requests, which is not normal: secondary read requests don't read
block from disk, only primary ones do.
I think it should be updated only close to pagecache_fread() calls.

global_cache_write seems ok: it is updated close to
pagecache_fwrite() calls.

How to repeat:
code review

[5 Mar 2008 10:10] Guilhem Bichot

BUG#34911 is not a duplicate but still interesting to know about

[12 Mar 2008 0:02] Bugs System

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/43780

ChangeSet@1.2616, 2008-03-11 18:12:08+02:00, [e-mail] +1 -0
  Fixed statistic calculation.
  (BUG#35030 maria_pagecache_read* status
  variables updated at wrong moment)