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

[Max Reitz]

On 19.11.2014 12:41, Paolo Bonzini wrote:
> On 19/11/2014 12:39, Prof. Dr. Michael Schefczyk wrote:
> > Dear Paolo,
> >
> > thank you very much for your prompt reply.
> Hi,
>
> please keep the replies on the mailing list.  I've added the qemu-devel
> mailing list, where more people should be able to see the message and
> hopefully reply with something helpful.
>
> Also, how do you do the backups?
>
> Paolo
>
> > For example, I have a guest named "gatewayb72.img" where the backup failed. If 
> > I thereafter try to create or delete a snapshot, the following reply occurs on the 
> > command line:
> >
> >
> > address@hidden kvm02]# qemu-img snapshot -d gatewayb72 /kvm02/gatewayb72.img
> > qemu-img: Could not open '/kvm02/gatewayb72.img': Could not read snapshots: 
> > File too large
> >
> >
> > If I want to reboot that machine, I get the following error:
> >
> >
> > Fehler beim Starten der Domain: Interner Fehler: Prozess wurde während der 
> > Verbindungsaufnahme zum Monitor beendet : qemu-kvm: -drive 
> > file=/kvm02/gatewayb72.img,if=none,id=drive-virtio-disk0,format=qcow2,cache=none:
> >  could not open disk image /kvm02/gatewayb72.img: Could not read snapshots: 
> > File too large
> >
> >
> > Traceback (most recent call last):
> >    File "/usr/share/virt-manager/virtManager/asyncjob.py", line 100, in 
> > cb_wrapper
> >      callback(asyncjob, *args, **kwargs)
> >    File "/usr/share/virt-manager/virtManager/asyncjob.py", line 122, in tmpcb
> >      callback(*args, **kwargs)
> >    File "/usr/share/virt-manager/virtManager/domain.py", line 1220, in startup
> >      self._backend.create()
> >    File "/usr/lib64/python2.7/site-packages/libvirt.py", line 698, in create
> >      if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
> > libvirtError: Interner Fehler: Prozess wurde während der Verbindungsaufnahme 
> > zum Monitor beendet : qemu-kvm: -drive 
> > file=/kvm02/gatewayb72.img,if=none,id=drive-virtio-disk0,format=qcow2,cache=none:
> >  could not open disk image /kvm02/gatewayb72.img: Could not read snapshots: 
> > File too large
> >
> >
> > Based on the facts I can see, the file is not too large. When reading the first error, 
> > the file size was 13.8 GB, while the limit is 14.5 GB. The same does also happen with 
> > files which are only, for example 6 GB big, while their limit is also 14.5 GB. Therefore, 
> > I think that the "file too large" really stands for something else.

It most probably does, yes. Some months ago, we included a limit in 
qemu's qcow2 implementation for the (internal) snapshot table size. When 
this limit is hit, "File too large" will be printed (it's a pretty bad 
error message, I know, but we thought it never to occur for reasonable 
images, so we did not see the need to improve it). That limit is 64 MB, 
which comes from having 1 kB of metadata per snapshot in average and a 
maximum of 64k snapshots. 1 kB per snapshot is reasonable: Normally, you 
have 56 bytes plus the length of the snapshot's name plus the length of 
the snapshot's ID (plus a couple of bytes of padding).

We have indeed seen some bug reports regarding this limit; however, 
nobody could provide us with an image to look into this issue so far (as 
far as I'm aware), and we could not reproduce it ourselves.

I understand that providing a 13.8 GB image is a bit much, but maybe you 
are able to do it anyway (the TU Dresden does have a reasonable internet 
connection, as far as I'm aware) or, if possible, provide us with 
information on how to recreate a similar image ourselves.

Max