[Libguestfs] Any risk in sparsifying a base image (that has a snapshot on top of it)

[Richard W.M. Jones]

On Tue, Nov 24, 2015 at 01:40:28PM +0000, Richard W.M. Jones wrote:
> On Tue, Nov 24, 2015 at 02:30:42PM +0200, Yaniv Kaul wrote:
> > Assuming the VM is not running, and we have a base (raw, sparse) with a
> > snapshot (qcow2) on top of it.
> > Is there any issue with running virt-sparsify on the base image? I assume
> > deleted blocks in the base can be sparsified, since they are either still
> > deleted on the snap (which is fine) or were written in the snap (which is
> > fine either and does not change or matter for the base image).
> 
> It could fail very badly (corrupting data), in at least the following
> situation:
> 
>  - Base image has Linux filesystems that require journal recovery
>    (eg. they were not unmounted, the VM was not shut down cleanly, or
>    they represent a snapshot of a running system)
> 
>  - virt-sparsify --inplace will do:
> 
>     for each fs:
>         mount fs /sysroot
>         fstrim /sysroot
> 
>  - The mount command will replace the journal, making arbitrary
>    changes to the base image.

s/replace/replay/

>  - The snapshot will not know about these changes, resulting in disk
>    corruption when you try to access the VM via the snapshot.
> 
> virt-sparsify copying mode would do something similar.
> 
> There may be other situations where it could fail.  Definitely don't do it :-)
> 
> > Can I assume sparsifying should not make any changes that matter for a VM
> > that will be spun (post the process)?
> 
> spun?
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> virt-top is 'top' for virtual machines.  Tiny program with many
> powerful monitoring features, net stats, disk stats, logging, etc.
> http://people.redhat.com/~rjones/virt-top

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top