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Re: [Qemu-devel] Evaluating Disk IO and Snapshots

From: André Braga
Subject: Re: [Qemu-devel] Evaluating Disk IO and Snapshots
Date: Thu, 19 Jan 2006 17:43:10 -0200

On 1/19/06, Juergen Pfennig <address@hidden> wrote:
> What I found is that qcow has poor performance. I wrote my own driver
> (which is intended only for -snapshot) and see signifcant improvements.
> A 300 MByte file copy (win2003 xcopy /e between two real drives) takes
> 90 instead of 135 seconds. I will send the patch to the list after it
> has matured for a while. The thing is linux only for mmap() is used.


While you are at it, have you considered using the LZO libraries
instead of zlib for compression/decompression speed? Sure, it won't
compress as much as zlib, but speed improvements should be noticeable.

I was thinking about doing this myself, but no doubt you now
understand the relevant source code on a level I'll still take a few
weeks to.

The source code for LZO is GPL'd, though. If I understand it
correctly, Mr. Oberhumer wouldn't mind doing an exception for QEMU and
licensing it as LGPL in this particular case; anyway, he must be
contacted to clarify this issue.

Link: http://www.oberhumer.com/opensource/lzo/

> The thing is linux only for mmap() is used.

On the link below you'll find some practical source code showing the
differences between **nix mmap() and Windows' equivalents:


Please consider using those so everyone can benefit ;)

Thank you!

"I decry the current tendency to seek patents on algorithms. There are
better ways to earn a living than to prevent other people from making
use of one's contributions to computer science."
Donald Knuth

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