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Re: [linux-lvm] Snapshots of snapshots are not supported yet



Dne 8.7.2013 19:57, markw mohawksoft com napsal(a):
Dne 8.7.2013 18:05, markw mohawksoft com napsal(a):
I'm trying to get a snapshot of a snapshot. I know the new thin
provisioning stuff supports this, but I'm not sure the release state is
something I can use at a customer.

Using LVM2, the snapshot format is tried and true, but inefficient if
you
want to take multiple snapshots.

Looking at dmsetup, there doesn't seem like there would be an issue
taking
a snapshot of a snapshot, but it is disabled in the tools.

I guess my question is this: Can it be done? Is there any reason why it
should not work?


1. Noone has written support for it in lvm2. And AFAICK noone is working
on this.

I thought I saw a patch for this in the 2010 time frame.


I'm not aware of any such support


2. Since the activation order of old snapshot is not as simple as with
other
targets, it's currently not so easy to add support for into lvm deptree
resolver.

In my case I would, more or less, be maintaining a lot of the
interdependency information for the chain of shapshots and it would
managed by a daemon being controlled by an application, not really subject
to the whims of a sys admin.


Beware the old snaps are meant to be used as a 'temporary  objects'
not something you want to keep for a long time - it's degrading write performance of origin device and there is also no safe resize support -
i.e. when you overfill snapshot - it's invalidated.  If you would
have used such snapshot as a base origin for a chain with not enough space - then whole chain is lost.

Is there a stable and supported way of getting this functionality? FYI:
I'm trying to use RHEL as the basis for this and I think that's an old
2.6.x kernel.

Kernel RHEL6 2.6 is a heavily modified version - so it has a lot
of features of 3.X kernel and dm support is basically back ported to very latest version - so you have there thin provisioning support.

If you want to use snaps of snaps - I'd really recommend to use
thins and report problems rather there - then trying to old snaps.
They were not designed for this use-case.

Zdenek



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