Dne 23. 10. 19 v 16:37 Gionatan Danti napsal(a):
On 23/10/19 14:59, Zdenek Kabelac wrote:
Dne 23. 10. 19 v 13:08 Gionatan Danti napsal(a):
Talking about thin snapshot, an obvious performance optimization which
seems to not be implemented is to skip reading source data when overwriting
in larger-than-chunksize blocks.
"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."
I would expect that such large-block *thin* snapshot rewrite behavior would
not cause a read/modify/write, but it really does.
Is this a low-hanging fruit or there are more fundamental problem avoiding
read/modify/write in this case?
Hi
If you use 1MiB chunksize for thin-pool and you use 'dd' with proper bs size
and you write 'aligned' on 1MiB boundary (be sure you user directIO, so you
are not a victim of some page cache flushing...) - there should not be any
useless read.
If you still do see such read - and you can easily reproduce this with latest
kernel - report a bug please with your reproducer and results.
Regards
Zdenek
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