[linux-lvm] exposing snapshot block device

Zdenek Kabelac zkabelac at redhat.com
Mon Nov 4 15:04:34 UTC 2019

Dne 04. 11. 19 v 15:40 Tomas Dalebjörk napsal(a):
> Thanks for feedback.

> Scenario 2: A write comes want to write block LP 100, but lvconvert has not 
> yet copied that LP block (yes, I do understand that origin is hidden now)
> Will lvconvery prioritize to copy data from /dev/vg00/lv00-snap to 
> /dev/vg00/lv00 for that block, and let the requestor write the changes 
> directly on the origin after the copying has been performed?
> Or will the write be blocked until lvconvert has finished the copying of the 
> requested block, and than a write can be accepted to the origin?
> Or where will the changes be written?

Since the COW device contains not only 'data' but also 'metadata'  blocks
and during the 'merge' it's being updated so it 'knows' which data has
been already merged back to origin (in other words during the merge the usage 
of COW is being reduced towards 0)  - I assume your 'plan' stops right here
and there is not much point to explore how much sub-optimal the rest of 
merging process is  (and as said - primary aspect was robustness - so if there 
is crash in any moment in time - data remain correct)



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