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RE: [linux-lvm] Restore LVM snapshot without creating a full dumpto an "external" device?



That is almost exactly what I was after.

Just one thing though... If I was to just take a backup of the COW
device, assuming the underlying origin had not changed, would I just be
able to restore this? (i.e. gzip < /path/to/cowdevice >
/path/to/backupfile.gz and restore with zcat /path/to/backup/file.gz >
/path/to/cowdevice, or similar.) Would this work with a newly created
snapshot? (ie backup as above then lvcreate -s -n newsnapshot
/path/to/origin; zcat /path/to/backup/file.gz > /path/to/new/cow/device)

I will just emphasize that the UNDERLYING ORIGIN WILL NEVER CHANGE, it
is never mounted, never used for anything except as a snapshot origin.

-----Original Message-----
From: linux-lvm-bounces redhat com [mailto:linux-lvm-bounces redhat com]
On Behalf Of Bas van Schaik
Sent: 10 March 2008 09:59
To: LVM general discussion and development
Subject: Re: [linux-lvm] Restore LVM snapshot without creating a full
dumpto an "external" device?

Lars Ellenberg wrote:
> On Sun, Mar 09, 2008 at 11:05:45PM +0100, Bas van Schaik wrote:
>   
>> Hi all,
>>
>> When I started to use LVM snapshots, I presumed that it was easy to
>> restore a system to such a snapshot. As far as I can see now, this
>> presumption was incorrect... People on the internet write that I
should
>> dump the whole snapshot using dd and then write it over the original
>> volume. This actually implies that I need another device with at
least
>> the size of the original volume available to dump to. In my
situation,
>> this means that I need about 2 TB free space to recover this
snapshot!
>>
>> Isn't there a more sophisticated way to restore the snapshot than
just
>> dumping it?
>>  1) create snapshot of /dev/myvolumegroup/myvolume to
>> /dev/myvolumegroup/mysnapshot
>>  2) dd if=/dev/myvolumegroup/mysnapshot of=/tmp/mysnapshot.dd
>>  3) lvremove /dev/myvolumegroup/mysnapshot
>>  4) dd if=/tmp/mysnapshot.dd of=/dev/myvolumegroup/myvolume
>>     
>
> you got (size-of-your-volume) free space in /tmp?
> pretty large /tmp, or pretty small volume, I guess.
>
>   
>> Something like:
>>  1) lvrevert /dev/myvolumegroup/mysnapshot
/dev/myvolumegroup/myvolume
>>
>> I'd like to hear your thoughts on this, because I think it should be
>> fairly easy to restore a COW snapshot. Or am I wrong and missing
something?
>>     
>
> you may want to investigate the status of
> http://fedoraproject.org/wiki/StatelessLinux/CachedClient
> were it says "The LVM and device-mapper code to allow merging is
> awaiting upstream review."
>   
Interesting project, interesting information, but it was last updated
2007-10-19. I'll try Googling for a more recent status, if that is
available...

> or you can try, at your own risk, the hack below.
>
> (... lots of excellent details ...)
Sounds very plausible, but there are some risks involved here ;). Thanks
anyhow, I'll really consider using your perl script when the situation
gets critical here...

  -- Bas

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