[linux-lvm] exposing snapshot block device

[Ilia Zykov]

On 23.10.2019 14:08, Gionatan Danti wrote:
> 
> For example, consider a completely filled 64k chunk thin volume (with
> thinpool having ample free space). Snapshotting it and writing a 4k
> block on origin will obviously cause a read of the original 64k chunk,
> an in-memory change of the 4k block and a write of the entire modified
> 64k block to a new location. But writing, say, a 1 MB block should *not*
> cause the same read on source: after all, the read data will be
> immediately discarded, overwritten by the changed 1 MB block.
> 
> However, my testing shows that source chunks are always read, even when
> completely overwritten.

Not only read but sometimes write.
I watched it without snapshot. Only zeroing was enabled. Before wrote
new chunks "dd bs=1048576 ..." chunks were zeroed. But for security it's
good. IMHO: In this case good choice firstly write chunks to the disk
and then give this chunks to the volume.
> 
> Am I missing something?
> 


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