Possible to use multiple disk to bypass I/O wait?

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?

Re: Possible to use multiple disk to bypass I/O wait?

On 9 June 2011 10:24, 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 an=
d
> 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 smar=
t
> enough to read using the other disk while the first's tied up with th=
e
> first process?
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> the body of a message to majordomo@vger.kernel.org
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ml
>

The first thing that comes to my mind: Have you tried another IO schedu=
ler?

/M
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Re: Possible to use multiple disk to bypass I/O wait?

----- Message from centos.admin@gmail.com ---------
Date: Thu, 9 Jun 2011 17:24:23 +0800
From: Emmanuel Noobadmin
Subject: Possible to use multiple disk to bypass I/O wait?
To: CentOS mailing list , linux-raid



> 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?

Have you tried using ionice -c 3 on the process?


----- End message from centos.admin@gmail.com -----



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Re: Possible to use multiple disk to bypass I/O wait?

On 6/9/11, Mathias Bur=E9n wrote:
> The first thing that comes to my mind: Have you tried another IO sche=
duler?

and the first thing that came to this noob's mind was: Wait, you mean
there's actually more than one? AND I get to choose?

I'll probably be experimenting with deadline and anticipatory since
the i/o wait seems to be due to the disk running back and fro trying
to serve the file scan as well as legit read request so having that
small wait for reads in the same area sounds like it would help.
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Re: Possible to use multiple disk to bypass I/O wait?

On Thu, 9 Jun 2011, 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.

If the server is reduced to a crawl, it's possible that you are hitting
the dirty_ratio limit due to writes and the server has entered synchronous
I/O mode. As others have mentioned, setting noatime could have a
significant effect, especially if there are many files and the server
doesn't have much memory. You can try increasing dirty_ratio to see if it
has an effect, eg:

# sysctl vm.dirty_ratio
# sysctl -w vm.dirty_ratio=50

Steve