[linux-lvm] LVM and chain of snapshots

Zdenek Kabelac zkabelac at redhat.com
Wed Jan 20 14:20:25 UTC 2016

Dne 20.1.2016 v 13:54 Марк Коренберг napsal(a):
> 2016-01-20 17:09 GMT+05:00 Zdenek Kabelac <zkabelac at redhat.com
> <mailto:zkabelac at redhat.com>>:
>     ed sta
> Thanks for the response, but I do not understand how thin-provisioning is
> related to question i'm asking.
> As far as I understand, if 20 snapshots are created even in thin-provisioning
> mode, write to origin will be converted
> to 21 writes. Does not it ? My scenario mean that no write multiplication
> occurs while making "normal" operations in
> userspace (i.e. not writing to snapshots, while origin is under heavy
> write-load). Also, my scenario adds functionality
> of "snapshot of snapshot" easily. The case I'm trying to discuss is something
> like chain of qcow2 files used to make
> live snapshots in KVM.

You cannot chain old-snaps this way (you cannot map old-snap over old-snap)
And no - it's not easy to  add one.
So your proposal would have worked for exactly 1 level
e.g.  you continue to write to snap - and you keep origin intact,
but you cannot map another 'snap' over this snap.

lvm2 is currently incapable of doing this - and it's fairly nontrivial to 
support this - and especially when we have thin-provisioning, noone is 
currently planning to extend old snapshot with such complicated feature.

> Use case: having such snapshot every day. And after snapshot count exceed 30,
> meld first snapshot into it's origin.

So you would need 30 chained snaps....
And after 30 of them - you actually would need merging them - quite complicated...

> This operation should be possible without any unmounting. After merging, that
> snapshot should contain empty diff
> and so may be eliminated from chain via replacing dmsetup tables.

It's much better to directly update 'origin' and just drop no longer needed 

The only major issue is - running  30 old snapshot it just not suitable for 
any use...

> In other words, my proposal is not connected to low-level things in LVM. Yes,
> all snapshots I describe can be
> thin-provisioned. Just minimal logic, CLI and XML should extended.

In other words - you just described how thin-provisioning works
and there is no reason to reinvent the wheel again :)

So please for this use-case switch to thin-provisioning.



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