Re: [Xen-users] High IOWAIT

[Maarten Vanraes <maarten@xxxxx>]

Op vrijdag 6 juni 2008 15:11, schreef Nick Anderson:
> Lorenzo Milesi wrote:
> > Hi
> >
> > I've a setup with opensource xen3.0.3 on debian etch.
> > lately i'm experiencing problems with the main debian domu, which runs
> > samba cyrus apache and few other services.
> > i'm having very high machine load due to iowait at 100%. i suppose the
> > culprit to be cyrus, but i'm not sure. i have 4.5g mail, split in
> > something like 100000 files. I already remounted my partition with
> > noatime,nodiratime as suggested by the cyrus site, but didn't help. The
> > vm has only 1 virtual disk of 100gb, 70% free, on lvm.
> > is there anything xen could be involved in?
>
> I would guess its imap thats causing your problem. What kind of disk
> back end is the vm on? I know you said lvm but is that lvm on a hardware
> raid, software raid, if so how many disks and what raid level?
>
> Ive had a web/mail server in the past that iowait was just tearing up
> performance. I migrated imap mail off to a different machine and that
> made a *huge* difference. That was just a single disked server with no
> hardware or software raid.
>
> I know people have said don't run a database or email in a vm but it
> seems to me all that needs to be addressed is having a fast storage back
> end. I hope I am right, as I plan to migrate our database and mail
> servers to virtual machines. Initially they will be on the same piece of
> hardware but eventually they will on separate dom0s. I know in the
> meantime there will be some IO contention but I am hoping that I can
> mitigate some of that with relatively fast IO back end and a lot of RAM.

I must say that we have great problems with extremely high IOWAIT, possibly 
due to the raid card not being fully hardware raid (allthough we had 
requested it).

We even used the tap:aio, but it made no difference.

I hope the new card will solve our problems, but maybe xen has a problem too.

our machines don't even use that much disk in the first place...
-- 
Maarten Vanraes
BA NV: IT & Security

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