This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-users] tap:aio Performance

To: support@xxxxxxxxxxxxxxx
Subject: Re: [Xen-users] tap:aio Performance
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Wed, 14 Feb 2007 12:06:21 +0000
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Delivery-date: Wed, 14 Feb 2007 04:05:51 -0800
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <45D24928.7080804@xxxxxxxxxxxxxxx>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
References: <45D24928.7080804@xxxxxxxxxxxxxxx>
Reply-to: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.4.1i
On Wed, Feb 14, 2007 at 12:26:32PM +1300, Peter wrote:
> Originally we had tried 3.0.4-0 with loop back mounted file systems.
> For some reason the dom0 crashed (after running for a day or so).  It
> did this a couple of times on one host server for us, and again once on
> another server.
> We have just tried 3.0.4-1 with tap:aio file systems on the domUs. 
> After that we've gone a couple of days with no kernel crash.  Good so far.

You don't say whether the underlying file you are pointing to is sparse
or pre-allocated (non sparse) ?  In the sparse case it is expected that
performance is terrible - because every write requires the undering FS
to allocate some more blocks - which in turn causes a journal sync. If
you use non-sparse then all the blocks are pre-allocated so you don't get
the journal bottleneck.

> However it seems that performance is a lot slower.
> e.g. on a domU:
> :/$ time sudo du -s
> 2020684 .
> real    9m25.646s
> user    0m0.044s
> sys     0m0.144s
> On a laptop with a puny 5400 rpm drive: $ time sudo du -s
> 86923472        .
> real    0m18.376s
> user    0m0.532s
> sys     0m10.737s
> And things like bonnie seem to make no real progress.
> And on startup things 'seem' slower.

Slower than what?  I'd certainly expect tap:aio: to be slower than file:
because file: is not actually flushing your data to disk - it hangs around
in memory and is flushed by the host kernel VM as needed. Since this isn't
remotely safe for your data, its not even worth comparing tap:aio with file:.

> Has anyone else experienced slower disk IO with 3.0.4/blktap?

Yes, when using sparse files ontop of a journalled fs.

|=- Red Hat, Engineering, Emerging Technologies, Boston.  +1 978 392 2496 -=|
|=-           Perl modules: http://search.cpan.org/~danberr/              -=|
|=-               Projects: http://freshmeat.net/~danielpb/               -=|
|=-  GnuPG: 7D3B9505   F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505  -=| 

Xen-users mailing list

<Prev in Thread] Current Thread [Next in Thread>