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



Dne 10.7.2013 05:45, markw mohawksoft com napsal(a):
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?


Yes - differential snapshot will be supported through thin provisioning
target - where you will be able to make a simple diff just by reading
metadata - it will be essential piece of replication.

AFAIK Joe has this in plan for some time - and there was even some announcement from some third-party developer to support this.

There is not going to be any upstream support for doing this with
old-snaps in foreseeable future.

Also keep in mind your idea of using old-snap of snap of snap would be very slow and fragile to use.

Zdenek


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