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Re: [Qemu-devel] [PATCH] bdrv_flush error handling

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] bdrv_flush error handling
Date: Wed, 20 Feb 2008 13:01:33 -0600
User-agent: Thunderbird (X11/20080214)

Daniel P. Berrange wrote:
On Wed, Feb 20, 2008 at 04:31:56PM +0000, Jamie Lokier wrote:
Ian Jackson wrote:
Paul Brook writes ("Re: [Qemu-devel] [PATCH] bdrv_flush error handling"):
Disk full is a fundamentally unfriendly situation to be in. There is no good answer. Reporting errors back to the host has its own set of problems. Many guest OS effectively just lock up when this occurs.
I think that's fine, surely ?  A locked up guest isn't very nice but
it's better than a guest shot dead.
Well, a guest which receives an IDE write error might do things like
mark parts of the device bad, to avoiding writing to those parts.  If
the guest is running software RAID, for example, it will radically
change its behaviour in response to those errors.

Sometimes that's what you want, but sometimes it is really unwanted.
If the host runs out of disk space, ideally you might want to suspend
the guest until you can free up host disk space (or move to another
host), then resume the guest, perhaps manually.

Is savevm-upon-disk-full a realistic prospect?

In the 'out of disk space' scenario you wouldn't need to save the guest - merely
stop its CPU. This gives the host admin the opportunity to hot-add storage
to the host & then resume execution, or to kill the VM, or to free enough
space to save the VM to disk / live migrate it to another host.

I agree. Stopping the CPUs and spitting out a big fat warning message would be the best thing to do. For the average user, this would give the opportunity to free up some space if possible.


Anthony Liguori

Shooting it dead on any I/O error doesn't give the host admin any choices
at all


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