Re: [linux-lvm] snapshotting to a different volume group

[Heinz Mauelshagen <mauelshagen@redhat.com>]

On Thu, Apr 17, 2008 at 03:58:09PM +0100, Stephane Chazelas wrote:
> Hiya,
> 
> I hope you could help me on this:
> 
> 
> in short:
> 
> is it possible to create a snapshot LV of an original LV
> but in a different volume group?

No, not with LVM.

> 
> or alternatively, what's the best way to bring up a LV in a VG
> that is missing a PV (as in a disk crash) if that LV has all its
> extents on still sane PVs?

vgchange -P -ay

If you have 2 disks, you better mirror them to be completely
HA on the spindle side of things and avoid any such config hassle,
potentially leaving you with lost data.

Heinz

> 
> 
> in long:
> 
> basically, this is what I would like to do:
> 
> I have two disks, not as much as I would like, and I want:
>   - to have 100GB of data mirrored (using Linux software raid
>     (mdadm)) on 2 disks so that it survives disk crashs
>   - to make a number of frequent snapshots of that data to
>     be able to restore things in case of human errors.
>   - (and I also want to have some "striped" storage space for
>     worthless data).
> 
> BUT, I don't want to have the snapshot volumes mirrored.
> 
> My initial idea was to have 2 volume groups: a "safe" one with
> only one /dev/md0 PV and an "unsafe" one with the striped LVs
> and the snapshots.
> 
> However I couldn't find how to create snapshots of the LVs in
> "safe" into the "unsafe" VG. It seems to me that it should be
> possible, shouldn't it?
> 
> Anyway, then I merged the two "safe" and "unsafe" VGs into one
> "vol" VG. Now I can make snapshots of the LV physically located
> on /dev/md0 into the "unsafe" area.
> 
> But if one of the disks fails, /dev/md0 will still be available
> but the "vol" VG won't be able to come up because of the other
> missing PVs.
> 
> If such a thing happen, I can happily live with the missing
> snapshots and "unsafe" storage for a while until I buy a
> replacement HD, but how should I do about making sure that the
> /dev/md0 filesystem is still available in the mean time?
> 
> Or is there a more straightforward way to achieve this that I
> haven't thought of?
> 
> TIA,
> Stephane
> 
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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Storage Development                               56242 Marienrachdorf
                                                  Germany
Mauelshagen@RedHat.com                            PHONE +49  171 7803392
                                                  FAX   +49 2626 924446
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