[Qemu-devel] copy on write-but-no-change

[Lapo Luchini <lapo@lapo.it>]

I noticed that cow adds space also if the written data is the same as
the one it was there already.
Is there a reason why checking it would be bad/difficult/slow/other?
Simply no one had the will to code the check?

I ask because for shrinking drives I usually create big zero-filled
files... and this grows the qcow2 image like mad, and only a compressed
convert can properly shrink it back. OK, it is not to be done very very
often, but not copying those writes that don't actually change the data
seems like a good idea... don't know about the speed hit, though.

Another idea would be to detect "only zero" write blocks and simply
interpret them as "put a hole in the sparse file" instead of actually
writing data to disk.

Just the first idea that hit me after a few hours of qemu usage, don't
hit me with a brick if it has already been discussed to death: I
searched it in the gmane.org archives to no avail ;-)

-- 
Lapo Luchini
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