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Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error.

From: Ian Jackson
Subject: Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error.
Date: Mon, 19 Jan 2009 17:22:04 +0000

Anthony Liguori writes ("Re: [Qemu-devel] [PATCH v3] Stop VM on ENOSPC error."):
> Ian Jackson wrote:
> > Once again, this feature should be optional.
> Why?

Well, three reasons, one general and theoretical, and two practical
and rather Xen-specific.

The theoretical reason is that a guest is in a better postion to deal
with the situation because it knows its access patterns.  Often the
response to a failing write in a mission-critical system will be some
kind a fallback behaviour, which is likely to work.  Stopping the VM
unconditionally is not something that the guest can cope with.

The practical reasons are that we would want to retain existing
behaviour unless it was clearly broken (which we don't think it is),
and that we don't currently have any useful mechanism for reporting
and dealing with the problem.

Fundamentally I think we're seeing this different because of the way
that Xen uses qemu is contextually quite different to the
`traditional' qemu.  Traditionally qemu is used as a subprogram of
other tasks, as an interactive debugging or GUI tool, or whatever.

But in the Xen context, a Xen VM is not a `task' in the same way.
(Xen users make much less use of the built-in cow formats for this
reason, often preferring LVM snapshots or even deeper storage magic.)
We expect the VM to be up and stay up and if it can't continue it
needs to fail or crash.


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