Re: [Xen-users] disk access besk practice

["Todd Deshane" <deshantm@xxxxxxxxx>]

On Tue, Jan 6, 2009 at 12:58 PM, Brian Krusic <brian@xxxxxxxxxx> wrote:
> Hi all,
>
> While I've read some faqs, forums and Professional Xen Virtualization, I
> would like your take on this.
>

You should read Running Xen ;)

> I've 2 paravirtualized domUs running, each using tap:aio disk image located
> on a local 500GB raid.
>
> While performance seems fine both interactively and using benchmarks, is
> there a practical limit to the image size before I should start breaking it
> up?
>

If there is a hard limit on the file system for max file size that is
could be an issue.
Otherwise, it can really depend on usage, backup considerations, etc.

> I plan to build another dom0 box with a 24TB raid on it and hosting 2
> paravirtualized domUs, one of which will need 20TB.
>
> Should I break up the domU into 2 images, 1 for the OS and the other for
> storage needs?
>

This can be beneficial in a general sense, for backup purposes and
also performance
could be achieved, just like  with a non-virtualized system writing to
different physical disks.

> So my questions are;
>
> 1 - Whats a practical single disk image size?

Others may have experience with very large disks....

> 2 - Should I pre allocate all image space during domU creation or have it
> dynamically grow?
>

It depends on performance needed. Dynamically growing will have some
performance degradation. And the dynamically allocated ones will save
you a lot of space. It is a trade off. In practice, if breaking up, you could
have a mixture of disks, the performance crucial ones could be pre-allocated
and the less performance crucial, less used, could be dynamically grown
(aka sparse files).

Hope that helps some.

Cheers,
Todd

-- 
Todd Deshane
http://todddeshane.net
http://runningxen.com

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