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

[markw at mohawksoft.com]

One more annoying question, if you have the patience.

Suppose I create a thin provisioned volume, say disk0.

I take a snapshot of disk0 and name it disk0_snap0
After a number of changes I then take another snapshot, and call it
disk0_snap1

Is the thin provisioning stuff smart enough to realize that disk0_snap0
should be rewired to reference disk0_snap1 as its origin? It would look
like this:

disk0 -> disk0_snap1 -> disk0_snap0

What I would like to see is something like this:

disk0 -> disk0_snap2 -> disk0_snap1 -> disk0_snap0

Create disk0_snap3

disk0 -> disk0_snap3 -> disk0_snap2 -> disk0_snap1 -> disk0_snap0

Where all writes to disk0 result in CoW to disk0_snap3 and the remaining
snaps remain unchanged.

Here is the kicker, I want to perform a differential backup on disk0_snap3
and disk0_snap2.  Suppose I already recreated disk0_snap2 on some server.
I now want to update it to disk0_snap3. I need to get a block list from
disk0_snap2 and disk0_snap3, then generate a list of blocks needed to
permute snap2 to snap3.

Any info?


> Dne 8.7.2013 19:57, markw at mohawksoft.com napsal(a):
>>> Dne 8.7.2013 18:05, markw at 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
>
>
>