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

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.

- 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