A beautiful picture is sometimes better than words :
With standard HDDs (here in RAID 10), the innodb_io_capacity variable may have non expected effects.
You can see on this picture the result of a test on one of my server with only the replication thread activated. The value of the innodb_io_capacity variable was the single modification during the period.
So, don’t touch this parameter without a strong reason…
(Of course 30000 was a bit snooty)
[MAJ 2013-01-08] : I know 30k is a too high value for innodb_io_capacity, I used this value to make the graphe really eloquent.
Read this excellent post from Chris for more details :