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

From: Thomas Huth
Subject: [Bug 498523] Re: Add on-line write compression support to qcow2
Date: Thu, 19 Nov 2020 09:39:35 -0000

As far as I know, QEMU v5.1 now has support for compression filters,
e.g. by creating a qcow2 image with:

 qemu-img create -f qcow2 -o compression_type=zlib image.qcow2 1G

... so I think we can finally mark this ticket here as done.

** Changed in: qemu
       Status: Confirmed => Fix Released

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  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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