Dne 20.1.2016 v 13:54 Марк Коренберг napsal(a):
2016-01-20 17:09 GMT+05:00 Zdenek Kabelac <zkabe...@redhat.com
<mailto:zkabe...@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
snapshot.
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.
Regards
Zdenek
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