Thousands concurrent parallel read write accesses over tens of millions of small files is a terrible performance tuning problem for e-mail servers.
You must understand and fine tune all your infrastructure chain, following the previous articles for data storage and multipath on Debian 5.x Lenny.
We reduced the CPU I/O wait from 30% to 0,3% (XFS) and 5% (EXT3) with these combined previously undocumented file system tuning tips.
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