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Re: [Libvir] Proposal: libvirt should remove or rename a save file after a successful restore



On Mon, Jul 02, 2007 at 04:38:48PM +0100, Richard W.M. Jones wrote:
> Chris Lalancette found this problem:
> 
> https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=246105
> 
> <quote>
> Once an "xm restore" has successfully completed, the -save file that it 
> came from should really be deleted.  Restoring a domain a second time 
> from the same save file will result in disk corruption; the kernel VM 
> state (in the saved file) will be in an inconsistent state with respect 
> to what is actually on disk (in the domain disk file).  The only valid 
> use I can see for the -save file after a restore is possibly for some 
> crash analysis, but even that could be worked around by renaming the 
> save file after a success.  In particular, I just ran into this 
> situation (and I'm worried customers might do the same):
> [and then he goes on to describe a scenario in which you can commonly 
> hit this situation]
> </quote>
> 
> So the obvious and simple solution, it seems to me, is just to change 
> libvirt.c:virDomainRestore so that if the driver underneath it returns 
> from the restore without error, then we either remove or rename the 
> restore file.
> 
> It's a trivial patch -- what do people think?

> My thoughts:
> * Removing the file might seem quite abrupt/unexpected, and there is the 
> possibility of data loss.

That would be placing  policy into the API which IMHO is wrong. Policy
should be in tools instead. So making virsh/virt-manager remove the
file is good, but we shouldn't force it on all users of the API. It
is quite possible that a user wishes to restore from the same file
mulitple times - assuming they know the caveats about snapshotting the
filesystem to match their VM state.

> * You have to rename the file as a hidden file (dotfile) in order for 
> xendomains to ignore it.  However these files are big and having large, 
> hidden files which build up in number over time sounds like it could be 
> a bad thing.

I don't like the idea of renaming just to get around xendomains issues.
Ad-hoc saved VMs should not be put in the same directory that xendomains
uses for its  managed saved-on-shutdown process. This bug should should
fixed separately.

> * This might be considered a significant behaviour change.
> * It's easier to handle this for the remote case if we make the change 
> inside libvirt, rather than in virsh etc.

We can't change the existing API semantics in this way. At best we could
add an additional variant of the  virDomainRestore which added a 'flags'
parameter perhaps.

Longer term we need to consider expanding the save/restore APIs because
both QEMU & Xen now have an idea of 'managed save images' as well as 
external save images.  WIth managed images, you don't supply a filename,
instead you simply tell it to save the VM (optionally with a 'tag' to
identify the image). XenD / QEMU can list previously saved images, and
optionally restore from one, or delete one. This supports the idea of
snapshots/checkpoints too. So I think we'll ultimately need several more
APIs in this area. The managed save/restore APIs will end up being separate
from the existing unmanaged APIs though.

Dan.
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