Re: [Qemu-block] [RESEND] RFC chaning zlib windowBits for QCOW2

[Stefan Hajnoczi]

On Mon, Jul 27, 2015 at 2:58 PM, Peter Lieven <address@hidden> wrote:
> I was experimenting a little with optimized zlib versions (like zlib-ng) to
> optimize the speed of compressed qcow2 generation.
>
> I found out that chaning the windowBits from -12 to -15 alone increases
> compression speed by about 15% while lowering
> the compressed size by about 6%. As a test case I used a Debian 8.1 disk
> image.
>
> What is your opinion in chaning the windowBits? The cost is a higher memory
> usage. The usage changes from 272k -> 384k for
> compression and from 4k -> 32k for decompression.

Kevin is the qcow2 maintainer, but judging purely by the numbers you
posted it seems attractive to make this change.

Does it have any impact on the file format?  Right now -12 is
hard-coded in QEMU, so we need to make sure that existing files and
programs that can access qcow2 files continue to work with both
old/new images.  If compatibility is not possible then you'll have to
investigate feature bits.

Stefan