Re: [Xen-users] xen 4.4 on shared disk (NFS)

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-10-21 at 16:47 +0200, Agustin Lopez wrote:
> Hello!
> 
> I am using Xen 3.x and 4.1 on Debian systems over several years. All Ok 
> with 20 Domains0
> and more of 100 DomUs.
> I am using a lock system among several Dom0s, using xend and 
> xend-domain-lock*
> options of the xend-config.sxp, and some patches I found in Internet.
> 
> Now I want to upgrade to last Debian Xen release, 4,4, and new 
> tool-stack xl (or libvirt).
> But with this upgrade my lock system can not work.
> 
> Are there any way of lock DomUs among several Dom0s with images in a 
> shared NFS ?

I'm afraid this previously out of tree functionality has not been
implemented in (lib)xl, to do so would require someone to author a similar
patch to the one you have been applying to xend but applying to the new
toolstack instead.

The new toolstack is largely written in C, if you have some familiarity
with that and would like advice etc on how to approach implementing this
functionality and getting it applied to the xen source then please to
approach the xen-devel list to discuss it first. It may be that this
usecase is better addressed in some other way (e.g. per disk image locks
rather than whole domain locks is something worth disucssing).

FWIW I don't think any particularly advanced C skills will be required, the
tricky bit IMHO will be in the POSIX locking semantics (particularly once
NFS is in the picture).

If this https://build.opensuse.org/package/view_file/home:nick_at_seakr:xcp
-xapi/xen/xend-domain-lock.patch (or something like it) is the patch which
you have been using then I must say that some of the caveats in the
comments wrt locking on an NFS share (plus my own experience with locking
on NFS) seem rather worrying to me. If this was to be implemented in
(lib)xl then I think we'd probably want to review the lock strategy before
starting down the path of implementation.

Otherwise, you might be able to write some sort of wrapper around xl which
takes the necessary locks, since there is one xl process per domain (unlike
the single process xend) so this might be workable.

Ian.

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