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Re: [linux-lvm] Temporary shapshots



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 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 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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