[linux-lvm] restoring snapshots

[linux-lvm] restoring snapshots

[Stephane Chazelas]

Hiya,

I was wondering about how to restore/apply a LVM snapshot to the
original partition. Looking at the archive, I can see it's
advised to dump the snapshot content into a file and then dump
that onto the original device after having removed the snapshot.

What's wrong with:

lvcreate -n lv -L1G vg
...
mount /dev/vg/lv /x
...
lvcreate -s -n lv_snap /dev/vg/lv
...
mount /dev/vg/lv_snap /y

Them to apply the snapshot:

umount /x /y
dd bs=4M < /dev/mapper/vg-lv_snap > /dev/mapper/vg-lv-real
lvremote /dev/vg/lv_snap

It's still not optimal as it writes the whole volume, but at
least, I don't have to find a storage area big enough to hold an
extra copy of the whole volume and it's faster.

It seems to be working on a test on /dev/loop. Is there any
circumstance (as long as there's only one snapshot of the
volume) where it wouldn't work, or what would be the potential
pitfalls (other than if there's a crash in the middle, you're
screwed)?

Is there any plan to add the feature to efficiently
restore/apply/commit a snapshot (that is only dump the cow data
onto the original volume without writing the whole volume) or
does that feature exist already and I missed it?

Thanks,
Stephane

Re: [linux-lvm] restoring snapshots

[Kinzel, David]

>Hiya,
>
>I was wondering about how to restore/apply a LVM snapshot to the
>original partition. Looking at the archive, I can see it's
>advised to dump the snapshot content into a file and then dump
>that onto the original device after having removed the snapshot.
>
>What's wrong with:
>
>lvcreate -n lv -L1G vg
>...
>mount /dev/vg/lv /x
>...
>lvcreate -s -n lv_snap /dev/vg/lv
>...
>mount /dev/vg/lv_snap /y
>
>Them to apply the snapshot:
>
>umount /x /y
>dd bs=4M < /dev/mapper/vg-lv_snap > /dev/mapper/vg-lv-real
>lvremote /dev/vg/lv_snap
>
>It's still not optimal as it writes the whole volume, but at
>least, I don't have to find a storage area big enough to hold an
>extra copy of the whole volume and it's faster.
>
>It seems to be working on a test on /dev/loop. Is there any
>circumstance (as long as there's only one snapshot of the
>volume) where it wouldn't work, or what would be the potential
>pitfalls (other than if there's a crash in the middle, you're
>screwed)?
>
>Is there any plan to add the feature to efficiently
>restore/apply/commit a snapshot (that is only dump the cow data
>onto the original volume without writing the whole volume) or
>does that feature exist already and I missed it?

The newer versions do have the lvmerge feature, and it seems to work in
RHEL6 beta from what I could see.

>
>Thanks,
>Stephane
>
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>linux-lvm@redhat.com
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>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

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Re: [linux-lvm] restoring snapshots

[Mike Snitzer]

On Mon, Sep 20 2010 at  9:15am -0400,
Kinzel, David <David.Kinzel@encana.com> wrote:

> >Is there any plan to add the feature to efficiently
> >restore/apply/commit a snapshot (that is only dump the cow data
> >onto the original volume without writing the whole volume) or
> >does that feature exist already and I missed it?
> 
> The newer versions do have the lvmerge feature, and it seems to work in
> RHEL6 beta from what I could see.

Yes, and the changes are also upstream, see:
https://www.redhat.com/archives/linux-lvm/2010-February/msg00029.html
https://www.redhat.com/archives/linux-lvm/2010-May/msg00017.html

Mike