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* [Qemu-devel] copy on write-but-no-change
@ 2007-09-06 22:55 Lapo Luchini
  2007-09-07 15:40 ` Luke -Jr
  0 siblings, 1 reply; 3+ messages in thread
From: Lapo Luchini @ 2007-09-06 22:55 UTC (permalink / raw)
  To: qemu-devel

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
lapo@lapo.it (OpenPGP & X.509)
www.lapo.it (Jabber, ICQ, MSN)

^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2007-09-11  6:47 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2007-09-06 22:55 [Qemu-devel] copy on write-but-no-change Lapo Luchini
2007-09-07 15:40 ` Luke -Jr
2007-09-11  6:47   ` Rob Landley

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