F20 System Wide Change: Visible Cloud

[Richard W.M. Jones]

On Mon, Jul 15, 2013 at 01:55:18PM +0200, Florian Weimer wrote:
> On 07/15/2013 01:47 PM, Daniel P. Berrange wrote:
> >Updating to qcow2 to support xz (or some other algo) as a built-in
> >compression choice would likely give better results.
> 
> Is the compressed image still seekable?  If yes, that constrains the
> window/block size and limits the gains from switching compressors.

Dan's answer covered one aspect of this: are qcow2 compressed
images seekable, answer: yes, of course!

But there's another answer for xz images.  Are these seekable?
Yes.  Surprisingly.  I've written the code for it:

https://rwmj.wordpress.com/2013/06/24/xz-plugin-for-nbdkit/#content

*However* you have to prepare the xz images correctly for this to
happen (using the --block-size argument, see blog posting above).
Images created using standard xz parameters won't be seekable.  I
suggest it's a good idea that we do that for the cloud images, since
the penalty is only about 1% and it makes them a lot more useful if
you can use them without uncompressing them.

nbdkit is included in Fedora.

Rich.

-- 
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