This could be useful for improving a poor performance at some environments:
"Low latency is a key factor to a good user experience.
With the Debian GNU / Linux 6.x kernel tuning, combined with the previous
articles hints about XenServer I/O latency, tuning filesystems, configuring
multipath, and data storage, we reduced eighty-four times our virtual block
device latency at our environment.
Thousands concurrent synchronous writes of random small files on filesystems
with millions of them while honoring write barriers for POSIX compliance, stress
the limits of the data storage, file systems, kernel I/O, virtualization I/O,
hardware data paths.
These are our findings and Linux kernel tunings in Cyrus IMAP e-mail servers
aggregator under heavy load production since November 2011."