Dne 23. 10. 19 v 14:20 Ilia Zykov napsal(a):
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.
Yep - we are recommending to disable zeroing as soon as chunksize >512K.
But for 'security' reason the option it's up to users to select what fits the
needs in the best way - there is no 'one solution fits them all' in this case.
Clearly when you put a modern filesystem (ext4, xfs...) on top of thinLV - you
can't read junk data - filesystem knows very well about written portions.
But if you will access thinLV device on 'block-level' with 'dd' command you
might see some old data trash if zeroing is disabled...
For smaller chunksizes zeroing is usually not a big deal - with bigger chunks
it slows down initial provisioning in major way - but once the block is
provisioned there are no further costs....
Regards
Zdenek
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