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Re: [Qemu-devel] Crash: When Host HDD is full

From: Paul Brook
Subject: Re: [Qemu-devel] Crash: When Host HDD is full
Date: Thu, 12 Jul 2007 18:03:48 +0100
User-agent: KMail/1.9.7

> >> Qemu might freeze the guest when it gets -ENOSPC, and say, retry every
> >> second or wait for user input on the monitor.
> >
> > Better would IMHO be to report an IO error to the guest and allow that to
> > decide what to do. If you're bothered about robustness and reliability
> > then arbitrarily stopping the guest is not acceptable behaviour. There's
> > no guarantee that space will become available in a finite timeframe.
> I've considered that, and I'm not sure.  You will likely get a storm of
> I/O errors on ENOSPC; with several ways for disaster to strike:
> - the guest doesn't handle I/O errors well, and keeps writing.  some of
> the writes are overwrites so they hit the disk and data is corrupted

If an guest OS ignores IO write errors it's just plain broken.

> - the guest decides the disk is bad because it has too many errors and
> initiates some recovery procedure
> Stopping the guest at least guarantees nothing unexpected happens.  If 
> it's part of a managed solution we can output a message to the monitor
> which eventually finds its way to the operator.

I don't buy this argument.  If you don't want "unexpected" things to happen 
then the solution is simple: Make sure you never run out of disk space. 

The fact is that your (virtual) disk *is* broken at this point. The guest OS 
is in a much better position to decide on an appropriate course of action, 
either by retrying or some other recovery mechanism.

There are various error contitions that could be used, for example 


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