[libvirt] bug: try to take disk snapshot for LVM2 Volume

[shu ming]

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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