Brendan Gregg has a very good ongoing series of blog posts about
the importance of measuring latency at the layer that’s
appropriate for the question you are trying to answer. If you’re
wondering whether I/O latency is a problem for MySQL, you need to
measure I/O latency at the filesystem layer, not the disk layer.
There are a lot of factors to consider. To quote from his latest
This isn’t really a problem with iostat(1M) – it’s a great tool
for system administrators to understand the usage of their
resources. But the applications are far, far away from the disks
– and have a complex file system in-between. For application
analysis, iostat(1M) may provide clues that disks could be
causing issues, but you really want to measure at the file system
level to directly associate latency with the application, and to
be inclusive of other file system latency issues.
Someone should add Brendan’s feed to Planet MySQL. Here …[Read more]