[Bug 498523] Re: Add on-line write compression support to qcow2

[Max Reitz]

The compression filter can be used e.g. with -drive
driver=compress,file.driver=qcow2,file.file.filename=foo.qcow2.
However, it shouldn’t be used lightly, as it will only do the right
thing in very specific circumstances, namely every cluster that’s
written to must not be allocated already.  So writing to the same
cluster twice will not work.  (Which is why I was hesitant to merge this
filter, but in the end I was contend with the fact that it’s at least
difficult enough to use that unsuspecting users hopefully won’t
accidentally enable it.)

(It should be noted that this is not a limitation of the compression
filter, though, but of qcow2’s implementation (VMDK isn’t any better).
So technically qemu has the feature now, but qcow2 is still missing it.)

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https://bugs.launchpad.net/bugs/498523

Title:
  Add on-line write compression support to qcow2

Status in QEMU:
  Fix Released

Bug description:
  This is a wishlist item.  Launchpad really need a way for the
  submitter to indicate this.

  It would be really cool if qemu were to support disk compression on-
  line for writes.

  I know this wouldn't be really easy.  Although most OS's use blocks,
  you can really only count on being able to compress 512-byte sectors,
  which doesn't give much room for a good compression ratio.  Moreover,
  the index indicating where in the image file each sector is located
  would be complex to manage, since the compressed blocks would be
  variable sized, and you'd be wanting to do some kind of best-fit
  allocation of space in the image file.  (If you were to make the image
  file compressed block size granularity, say, 64 bytes, you could
  probably do this best fit O(1).)  If you were to buffer enough writes,
  you could group arbitrary sequences of written sectors into blocks to
  compress (which with writeback could be sent to a helper thread on
  another CPU, so the throughput would be good).

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