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

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH] bdrv_flush error handling
Date: Wed, 20 Feb 2008 18:22:52 +0000
User-agent: Mutt/1.4.1i

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. 

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

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