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[Qemu-devel] [Bug 498523] Re: Add on-line write compression support to q
From: |
Paul Maunders |
Subject: |
[Qemu-devel] [Bug 498523] Re: Add on-line write compression support to qcow2 |
Date: |
Sat, 25 Aug 2012 10:10:37 -0000 |
+1 vote for this feature.
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https://bugs.launchpad.net/bugs/498523
Title:
Add on-line write compression support to qcow2
Status in QEMU:
Confirmed
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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