FAQ

On 06/09/2011 02:24 AM, Emmanuel Noobadmin wrote:
I'm trying to resolve an I/O problem on a CentOS 5.6 server. The
process basically scans through Maildirs, checking for space usage and
quota. Because there are hundred odd user folders and several 10s of
thousands of small files, this sends the I/O wait % way high. The
server hits a very high load level and stops responding to other
requests until the crawl is done.

I am wondering if I add another disk and symlink the sub-directories
to that, would that free up the server to respond to other requests
despite the wait on that disk?

Alternatively, if I mdraid mirror the existing disk, would md be smart
enough to read using the other disk while the first's tied up with the
first process?
You should look at running your process using 'ionice -c3 program'. That
way it won't starve everything else for I/O cycles. Also, you may want
to experiment with using the 'deadline' elevator instead of the default
'cfq' (see http://www.redhat.com/magazine/008jun05/features/schedulers/
and http://www.wlug.org.nz/LinuxIoScheduler). Neither of those would
require you to change your hardware out. Also, setting 'noatime' for the
mount options for partition holding the files will reduce the number of
required I/Os quite a lot.

But yes, in general, distributing your load across more disks should
improve your I/O profile.

--
Benjamin Franz

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