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Old 04-12-2008, 10:27 PM
"Joseph L. Casale"
 
Default merge an lvm snapshot back

So how does one accomplish this if say the snap is now deemed the copy of interest? I am hoping dd is not the only answer

Thanks!
jlc
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Old 04-13-2008, 12:59 AM
Karanbir Singh
 
Default merge an lvm snapshot back

Joseph L. Casale wrote:

So how does one accomplish this if say the snap is now deemed the copy of interest? I am hoping dd is not the only answer



well, just use it.

Also, your work flow is broken if your historic snapshots are now
production while the real physical LV isnt.( imho )



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Karanbir Singh : http://www.karan.org/ : 2522219@icq
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Old 04-13-2008, 08:04 AM
"William L. Maltby"
 
Default merge an lvm snapshot back

On Sun, 2008-04-13 at 01:59 +0100, Karanbir Singh wrote:
> Joseph L. Casale wrote:
> > So how does one accomplish this if say the snap is now deemed the copy of interest? I am hoping dd is not the only answer
> >
>
> well, just use it.
>
> Also, your work flow is broken if your historic snapshots are now
> production while the real physical LV isnt.( imho )

IIRC, you don't have a real snapshot on the LV ATM. The snapshot LV only
contains files that were chang(ed/ing) while the snapshot volume was
"attached" during the backup.

Don't trust me though - read the docs to be sure. Last time I looked it
might have been vers 1.

>
>

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Old 04-13-2008, 02:04 PM
Karanbir Singh
 
Default merge an lvm snapshot back

William L. Maltby wrote:

IIRC, you don't have a real snapshot on the LV ATM. The snapshot LV only
contains files that were chang(ed/ing) while the snapshot volume was
"attached" during the backup.


That does not mean you cant use the snapshot, as long as you are ready
to live with the fallouts of such a move ( and have enough space
allocated )


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Old 04-13-2008, 03:47 PM
"Bart Schaefer"
 
Default merge an lvm snapshot back

On Sat, Apr 12, 2008 at 5:59 PM, Karanbir Singh <mail-lists@karan.org> wrote:
> well, just use it.

This implies that "snapshot" is a bit of a misnomer for what LVM
creates, because you can modify both the "snapshot" and the "real
physical LV," but if you want to be able to revert easily, it's the
"real LV" that you have to *avoid* changing (so that you can simply
drop the snapshot when you're finished with it).

Have I got that wrong?

This limits the usefulness of snapshots as a backup/recovery mechanism.

> Also, your work flow is broken if your historic snapshots are now
> production while the real physical LV isnt.( imho )

Seems to me that's exactly the situation he's trying to avoid. He
wants to restore the real LV to the state it was in at the time one of
the snapshots was taken. Which in other contexts is exactly the
reason one makes a snapshot.
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Old 04-13-2008, 04:07 PM
"William L. Maltby"
 
Default merge an lvm snapshot back

On Sun, 2008-04-13 at 15:04 +0100, Karanbir Singh wrote:
> William L. Maltby wrote:
> > IIRC, you don't have a real snapshot on the LV ATM. The snapshot LV only
> > contains files that were chang(ed/ing) while the snapshot volume was
> > "attached" during the backup.
>
> That does not mean you cant use the snapshot, as long as you are ready
> to live with the fallouts of such a move ( and have enough space
> allocated )

Right. I started to say something about that earlier (I have an
automated b/u script I wrote way-back-when that takes advantage of the
snapshot facility) but decided that mentioning that to the OP might lead
to some unexpected hardships for him/her. I decided that just letting
h{im | er } know that it didn't have the full FS was a better choice of
information to pass on.

And in all honesty, it's been so long since I read up on it (vers 1,
IIRC) and looked at the script, I really didn't feel qualified to
suggest anything useful with any certainty.

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Old 04-13-2008, 04:09 PM
Karanbir Singh
 
Default merge an lvm snapshot back

Bart Schaefer wrote:

Also, your work flow is broken if your historic snapshots are now
production while the real physical LV isnt.( imho )


Seems to me that's exactly the situation he's trying to avoid. He
wants to restore the real LV to the state it was in at the time one of
the snapshots was taken. Which in other contexts is exactly the
reason one makes a snapshot.


Thats not what the whole point is. The point of doing snapshots is to
expose TO your backup process a consistent filesystem while reducing
the production downtime ( for people who use snapshots as a back
transport ). The Snapshot isnt meant to be the backup.


Anyway, if you are looking to discuss the in's and out's of lvm and how
it does what it does, there is a linux-lvm list, you might want to go
ask there.


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Old 04-13-2008, 04:15 PM
"William L. Maltby"
 
Default merge an lvm snapshot back

On Sun, 2008-04-13 at 08:47 -0700, Bart Schaefer wrote:
> On Sat, Apr 12, 2008 at 5:59 PM, Karanbir Singh <mail-lists@karan.org> wrote:
> > well, just use it.
>
> This implies that "snapshot" is a bit of a misnomer for what LVM
> creates, because you can modify both the "snapshot" and the "real
> physical LV," but if you want to be able to revert easily, it's the
> "real LV" that you have to *avoid* changing (so that you can simply
> drop the snapshot when you're finished with it).
>
> Have I got that wrong?
>
> This limits the usefulness of snapshots as a backup/recovery mechanism.
>
> > Also, your work flow is broken if your historic snapshots are now
> > production while the real physical LV isnt.( imho )
>
> Seems to me that's exactly the situation he's trying to avoid. He
> wants to restore the real LV to the state it was in at the time one of
> the snapshots was taken. Which in other contexts is exactly the
> reason one makes a snapshot.

At the risk of looking foolish now, I'll discourse a little from
*memory*. Then y'all can take KB's advice if desired.

IIRC, the snapshot volume holds changes to the base volume that occur
while it is mounted, leaving the base unchanged so that a (e.g.) backup
process sees a fixed state on the base file system. Only changes are on
the snapshot volume and I don't know at what level (changed blocks,
i-nodes, whole files or what). Regardless, when the snapshot volume is
unmounted, the changes are then commited to the base volume. But the
record of those things are still on the volume and it is mountable and
usable independently, implying that there might be complete files on
there. I don't know for sure.

Regardless, the snapshot volume is not a "mirror" of the base and is not
suitable for an unfiltered restore to the base and is not useful as a
full replacement for the base FS.

Someone please correct me if I'm wrong - it'll save me reading again!
=>8-O

> <snip sig stuff>

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Old 04-14-2008, 01:59 AM
"Joseph L. Casale"
 
Default merge an lvm snapshot back

>He wants to restore the real LV to the state it was in at the time one of
>the snapshots was taken.

Nah
I want to sanp a volume, mount the snap, do my `stuff`, if I like it, I can merge into the primary copy and continue or dump the snap and go back...

That would be so very usefull! I know I can dd out the snap but that so not elegant (and slow)!

jlc
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Old 04-14-2008, 04:23 AM
Garrick Staples
 
Default merge an lvm snapshot back

On Sun, Apr 13, 2008 at 07:59:20PM -0600, Joseph L. Casale alleged:
> >He wants to restore the real LV to the state it was in at the time one of
> >the snapshots was taken.
>
> Nah
> I want to sanp a volume, mount the snap, do my `stuff`, if I like it, I can merge into the primary copy and continue or dump the snap and go back...
>
> That would be so very usefull! I know I can dd out the snap but that so not elegant (and slow)!

Use rsync to copy the changes to the lv?

--
Garrick Staples, GNU/Linux HPCC SysAdmin
University of Southern California

Please avoid sending me Word or PowerPoint attachments.
See http://www.gnu.org/philosophy/no-word-attachments.html
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