Dne 22. 10. 19 v 18:15 Stuart D. Gathman napsal(a):
On Tue, 22 Oct 2019, Zdenek Kabelac wrote:
Dne 22. 10. 19 v 17:29 Dalebjörk, Tomas napsal(a):
But, it would be better if the cow device could be recreated in a faster
way, mentioning that all blocks are present on an external device, so that
the LV volume can be restored much quicker using "lvconvert --merge" command.
I do not want to break your imagination here, but that is exactly the thing
you can do with thin provisioning and thin_delta tool.
lvconvert --merge does a "rollback" to the point at which the snapshot
was taken. The master LV already has current data. What Tomas wants to
be able to do a "rollforward" from the point at which the snapshot was
taken. He also wants to be able to put the cow volume on an
extern/remote medium, and add a snapshot using an already existing cow.
This way, restoring means copying the full volume from backup, creating
a snapshot using existing external cow, then lvconvert --merge instantly
logically applies the cow changes while updating the master
LV.
Pros:
"Old" snapshots are exactly as efficient as thin when there is exactly
one. They only get inefficient with multiple snapshots. On the other
hand, thin volumes are as inefficient as an old LV with one snapshot.
An old LV is as efficient, and as anti-fragile, as a partition. Thin
volumes are much more flexible, but depend on much more fragile database
like meta-data.
Just few 'comments' - it's not really comparable - the efficiency of thin-pool
metadata outperforms old snapshot in BIG way (there is no point to talk about
snapshots that takes just couple of MiB)
There is also BIG difference about the usage of old snapshot origin and snapshot.
COW of old snapshot effectively cuts performance 1/2 if you write to origin.
For this reason, I always prefer "old" LVs when the functionality of
thin LVs are not actually needed. I can even manually recover from
trashed meta data by editing it, as it is human readable text.
On the other hand you can loose COW snapshot at any moment in time
if your 'COW' storage is no big enough - this is very different
from thin-poo.....
Regards
Zdenek
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