[linux-lvm] Temporary shapshots

Larry Dickson ldickson at cuttedge.com
Mon May 5 14:30:21 UTC 2008

I love this idea because it nicely covers one of the main purposes of
snapshotting, namely, making backups at a well-defined time point. Would
there ever be any use of a temporary read-write snapshot? If not, perhaps
making it read-only would simplify the design.

Larry Dickson
Cutting Edge Networked Storage

On 5/3/08, Stuart D. Gathman <stuart at bmsi.com> wrote:
> On Fri, 2 May 2008, Mikulas Patocka wrote:
> > To use temporary storage for snapshot, a special command for lvm would be
> > more appropriate --- a command that would setup snapshot and write
> nothing
> > about it to metadata, so that the snapshot would be forgotten on next
> > reboot --- then, you can setup the snapshot on any device outside the
> > volume group. You can already do this with dmsetup.
> This is an excellent idea - and not just for ramdisks.  The hardest part
> of using snapshots for backups is making sure they are deleted afterward.
> Not having to check at reboot would be one less 'i' to dot.
> Now supposing this feature is added to lvm - what if an enterprising
> admin decides to create *two* snapshots using the same block device?
> This could be a caveat emptor - or it could check that the block device
> was not in use by anything else, like another snapshot.
> Couldn't a LV be used for a temporary snapshot also?  It would just
> look like the COW instead of the snapshot image.  I could allocate
> a backup_cow LV for backup use only.
> The only problem I see is newbies getting very confused wondering why
> their "snapshot" won't mount.  That's why I suggest *not* calling the
> examples (of the temporary snapshot feature) in the docs anything with
> "snap"
> in the name.  I suggest "cow" as above.
> --
>              Stuart D. Gathman <stuart at bmsi.com>
>    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
> "Confutatis maledictis, flammis acribus addictis" - background song for
> a Microsoft sponsored "Where do you want to go from here?" commercial.
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