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Date: Mon, 20 Sep 2010 12:16:22 +0100
From: Stephane Chazelas <stephane_chazelas@yahoo.fr>
Message-ID: <chaz20100920111622.GA3547@yahoo.fr>
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Subject: [linux-lvm] restoring snapshots
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To: linux-lvm@redhat.com

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