Checking the source of the disk will exclude the case when the source disk is block device while the snapshot target is given manually by the users with "snapshot-create --xmlfile" or "virsh snapshot-create-as --diskspec".   Why not check the snapshot directory where it will be created?


On 2011-11-16 23:45, Eric Blake wrote: 
On 11/15/2011 08:20 PM, Dave Allan wrote:

After working on this some more, I think that identifying problematic
file systems, like devtmpfs, is too tricky to be portable.  But I think
we can meet halfway - right now, the libvirt code blindly generates a
filename if one was not provided, regardless of the file type of the
backing file it will be wrapping.  But maybe it would be sufficient to
make the command error out if no qcow2 filename is provided and the
backing file is not a regular file.  As in this patch:

Ok, now I understand the situation; thanks to everyone for the
explanations.  I'm somewhat uncomfortable using file type as a proxy
for "troublesome filesystem" since although they are linked in this
case, I'm not sure they're linked in all cases.  Maybe I'm wrong about
that.  If there is no portable way to determine whether a particular
filesystem is going to be a problem, I would just clearly document the
limitations of letting libvirt choose the filename and the importance
of not using that functionality on filesystems that cannot hold a
useful snapshot.

My patch would not be preventing snapshots of block devices, but merely
requiring that the user supply the qcow2 filename that will wrap the
block device.  The problem with just documenting the issue is that
someone will fail to read the documentation, perform a default snapshot
that creates a problematic qcow2 file, then be upset that their domain
fails to boot and that they lost all the changes made since the
snapshot.  I'm still in favor of this patch if anyone else thinks it is
worthwhile.


Subject: [PATCH] snapshot: refuse to generate names for non-regular backing
 files

For whatever reason, the kernel allows you to create a regular
file named /dev/sdc.12345; although this file will disappear the
next time devtmpfs is remounted.  If you let libvirt generate
the name of the external snapshot for a disk image originally
using the block device /dev/sdc, then the domain will be rendered
unbootable once the qcow2 file is lost on the next devtmpfs
remount.  In this case, the user should have used 'virsh
snapshot-create --xmlfile' or 'virsh snapshot-create-as --diskspec'
to specify the name for the qcow2 file in a sane location, rather
than relying on libvirt generating a name that is most likely to
be wrong.  We can help avoid naive mistakes by enforcing that
the user provide the external name for any backing file that is
not a regular file.

* src/conf/domain_conf.c (virDomainSnapshotAlignDisks): Only
generate names if backing file exists as regular file.
Reported by MATSUDA Daiki.


      

      

      

      
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