Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request)

From: "Prof. Dr. Michael Schefczyk" <michael@schefczyk.net>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request)
Date: Tue, 25 Nov 2014 11:49:21 +0000	[thread overview]
Message-ID: <2A6E6B95B6E5C146ACE8440760E58185BDC5FDB0@EXCHANGESERVER.schefczyk.local> (raw)
In-Reply-To: <20141125113405.GD4641@noname.redhat.com>

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Dear Kevin,

thank you very much! For the moment, I did stabilize my systems with these commands. For a next step, will explore three further options, which should all solve the issue better:

- move to ovirt altogether - probably as foolproof as needed at my skill level

- install ovirt's qemu-kvm-rhev to use the more extensive options

- move to QMP

Thanks again also to Paolo and Eric for providing substantial insights!

Regards,

Michael

-----Ursprüngliche Nachricht-----
Von: Kevin Wolf [mailto:kwolf@redhat.com] 
Gesendet: Dienstag, 25. November 2014 12:34
An: Prof. Dr. Michael Schefczyk
Cc: Eric Blake; Paolo Bonzini; qemu-devel
Betreff: Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request)

Am 22.11.2014 um 18:02 hat Prof. Dr. Michael Schefczyk geschrieben:
> Dear All,
> 
> after some trying, my impression is that the following steps do work with plain Centos 7:
> 
> virsh snapshot-create-as VM backsnap
> qemu-img convert -f qcow2 -s backsnap -O qcow2 VM.img backup.img virsh 
> snapshot-delete VM backsnap
> 
> Am I on the right track with these commands?

I won't tell you that you're bound to break your image with this sequence, because chances are that you won't break it. (In practice, this happens to work in most cases, because the essential snapshot metadata generally isn't written to without explicit user actions on that snapshot.)

But you're violating the rule of "one writer or many readers", so as far as qemu is concerned, we won't be careful to avoid breaking this setup.
Don't blame us if it fails one day.

With the QMP solutions (either drive-backup or snapshot/commit) you're using official APIs, so I'd encourage you to use these.

Kevin

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