[linux-lvm] Snapshot of a Snapshot

[linux-lvm] Snapshot of a Snapshot

[Karl Wagner]

Hi

I was just wondering if there was a plan to add 'Snapshot of a Snapshot'
functionality to LVM2? If so, is there any timescale on it?

This would be a very useful feature, especially in a Xen environment.

Also, is there an easy way to backup and restore a snapshot? My scenario
is that I am running several Xen DomU's (Virtual machines, 4 of them to
be precise) from snapshots of a base image. However, if one dies (eg the
snapshot overflows, something which should not happen again but just in
case...) and I recreate the snapshot and restore the backup image I
took, it fills the snapshot up. It doesn't seem to check if the data
being written to the snapshot is the same as on the origin volume, just
adds the data to the snapshot anyway. Is there any way round this (apart
from the very slow script I currently have which checks each sector
before writing)?

Cheers in advance


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Re: [linux-lvm] Snapshot of a Snapshot

[Dan Kegel]

On Fri, Feb 29, 2008 at 3:27 AM, Karl Wagner <kwagner@zetex.com> wrote:
>  I was just wondering if there was a plan to add 'Snapshot of a Snapshot'
>  functionality to LVM2? If so, is there any timescale on it?
>
>  This would be a very useful feature, especially in a Xen environment.
>
>  Also, is there an easy way to backup and restore a snapshot? My scenario
>  is that I am running several Xen DomU's (Virtual machines, 4 of them to
>  be precise) from snapshots of a base image. However, if one dies (eg the
>  snapshot overflows, something which should not happen again but just in
>  case...) and I recreate the snapshot and restore the backup image I
>  took, it fills the snapshot up.

It sounds like you could use a 'revert to snapshot' feature.
Is that right?
- Dan

Re: [linux-lvm] Snapshot of a Snapshot

[Karl Wagner]

Not quite, although this would also be nice. Maybe I should explain my
current setup better.

Firstly, I have a Xen DomU machine running several Dom0's. As each Dom0
is very similar, I started off with a Base install, then created
snapshots to modify to create each Dom0. This had the advantage of
allowing me to set up a new Dom0 very easily, while taking less physical
disk space (and also the side effect that it runs quicker, think that's
something to do with the block device cache being used for the origin,
but I'm no expert so it may not...)

What I would like to be able to do is:
(a) be able to take a snapshot of one of these snapshots in order to
make backups, or have a bookmark in time if I am doing something I am
not sure of (in this instance a 'revert to snapshot' would be nice too)
(b) if I need to restore a backup, it would be nice to just be able to
dd the image I took as backup onto the snapshot device, without it
copying on every write, only copying the sectors (or chunks, whatever
the allocation unit for the snapshot COW device is) which are different.

These would also be useful for the SAN I am putting together.

Both of these I can do manually, the first by creating a snapshot using
device-mapper, the second by comparing the sector between the current
and the backup before writing. Snapshot of a snapshot done this way is
very useful but a pain to set up, although I have written a script to do
this for me. Restoration of a snapshot this way is painfully slow
(although if functionality of this sort either is not going to be
implemented or will take a long time I will recode it in C to
(hopefully) speed things up, currently I am using a hack together of
bash and perl scripts, just to get it done quickly).

Regards
Karl

Date: Fri, 29 Feb 2008 06:39:50 -0800
From: "Dan Kegel" <dank@kegel.com>
Subject: Re: [linux-lvm] Snapshot of a Snapshot
To: "LVM general discussion and development" <linux-lvm@redhat.com>
Message-ID:
	<a71bd89a0802290639t34ee72e7q4379df51e392b900@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

On Fri, Feb 29, 2008 at 3:27 AM, Karl Wagner <kwagner@zetex.com> wrote:
>  I was just wondering if there was a plan to add 'Snapshot of a
Snapshot'
>  functionality to LVM2? If so, is there any timescale on it?
>
>  This would be a very useful feature, especially in a Xen environment.
>
>  Also, is there an easy way to backup and restore a snapshot? My
scenario
>  is that I am running several Xen DomU's (Virtual machines, 4 of them
to
>  be precise) from snapshots of a base image. However, if one dies (eg
the
>  snapshot overflows, something which should not happen again but just
in
>  case...) and I recreate the snapshot and restore the backup image I
>  took, it fills the snapshot up.

It sounds like you could use a 'revert to snapshot' feature.
Is that right?
- Dan


_________________________________________________________

Zetex Semiconductors - Solutions for an analog world.

http://www.zetex.com
http://www.zetex.cn

E-MAILS are susceptible to interference.  You should not assume that
the contents originated from the sender or the Zetex Group or that they 
have been accurately reproduced from their original form.
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this e-mail nor for its use or misuse nor for any act committed or
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Re: [linux-lvm] Snapshot of a Snapshot

[Tomasz Chmielewski]

Karl Wagner schrieb:
> Not quite, although this would also be nice. Maybe I should explain my
> current setup better.
> 
> Firstly, I have a Xen DomU machine running several Dom0's.

Just to use correct terms: in a Xen world, one runs domU (or, a guest) 
on top of dom0 (a.k.a host), not the other way around.

So you have a Xen dom0 running several domUs.


-- 
Tomasz Chmielewski
http://wpkg.org

Re: [linux-lvm] Snapshot of a Snapshot

[Karl Wagner]

> Just to use correct terms: in a Xen world, one runs domU (or, a guest)
on > top of dom0 (a.k.a host), not the other way around. 
>
> So you have a Xen dom0 running several domUs.

Yes, sorry, of course that's what I meant :)

Think my brain must have temporarily shut down when I wrote that.


_________________________________________________________

Zetex Semiconductors - Solutions for an analog world.

http://www.zetex.com
http://www.zetex.cn

E-MAILS are susceptible to interference.  You should not assume that
the contents originated from the sender or the Zetex Group or that they 
have been accurately reproduced from their original form.
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Re: [linux-lvm] Snapshot of a Snapshot

[Lars Ellenberg]

On Mon, Mar 03, 2008 at 09:13:05AM -0000, Karl Wagner wrote:
> Not quite, although this would also be nice. Maybe I should explain my
> current setup better.
> 
> Firstly, I have a Xen DomU machine running several Dom0's. As each Dom0
> is very similar, I started off with a Base install, then created
> snapshots to modify to create each Dom0. This had the advantage of
> allowing me to set up a new Dom0 very easily, while taking less physical
> disk space (and also the side effect that it runs quicker, think that's
> something to do with the block device cache being used for the origin,
> but I'm no expert so it may not...)
> 
> What I would like to be able to do is:
> (a) be able to take a snapshot of one of these snapshots in order to
> make backups, or have a bookmark in time if I am doing something I am
> not sure of (in this instance a 'revert to snapshot' would be nice too)
> (b) if I need to restore a backup, it would be nice to just be able to
> dd the image I took as backup onto the snapshot device, without it
> copying on every write, only copying the sectors (or chunks, whatever
> the allocation unit for the snapshot COW device is) which are different.
> 
> These would also be useful for the SAN I am putting together.
> 
> Both of these I can do manually, the first by creating a snapshot using
> device-mapper, the second by comparing the sector between the current
> and the backup before writing. Snapshot of a snapshot done this way is
> very useful but a pain to set up, although I have written a script to do
> this for me.

just make sure you never ever write to the origin accidentally
while having a snapshot of a snapshot of it.
if it was for an area that needed COW,
it would deadlock kcopyd. hard.
because kcopyd would recurse into itself from the
completion callbacks it was told to call back. :(

> Restoration of a snapshot this way is painfully slow
> (although if functionality of this sort either is not going to be
> implemented or will take a long time I will recode it in C to
> (hopefully) speed things up, currently I am using a hack together of
> bash and perl scripts, just to get it done quickly).

see my other post in that other thread
 "Restore LVM snapshot without creating a full dump"
maybe that script is of some use for you, too?

-- 
: Lars Ellenberg                            Tel +43-1-8178292-55 :
: LINBIT Information Technologies GmbH      Fax +43-1-8178292-82 :
: Vivenotgasse 48, A-1120 Vienna/Europe    http://www.linbit.com :