Re: [Qemu-devel] [PATCH] Align file accesses with cache=off (O_DIRECT)

[Laurent Vivier <Laurent.Vivier@bull.net>]

Le lundi 28 avril 2008 à 17:34 +0200, Kevin Wolf a écrit :
> Kevin Wolf schrieb:
> > In December a patch was applied which introduced the cache=off option to
> > -drive. When using this option files are opened with the O_DIRECT flag.
> > This means that all accesses have to be aligned. The patch made a couple
> > of changes in this respect, still in other places they are missing (e.g.
> > you can't use cache=off with qcow(2) files).
> > 
> > This patch implements wrappers for raw_pread and raw_pwrite which align
> > all file accesses and make qcow(2) work with cache=off. This method
> > might not be the most performant one (compared to fixing qcow, qcow2 and
> > everything else that might be using unaligned accesses), but unaligned
> > accesses don't happen that frequently and with this patch really all
> > image accesses should be covered.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@suse.de>
> 
> Anything wrong with this one?

Hi Kevin,

I did a patch to manage this problem too:

http://thread.gmane.org/gmane.comp.emulators.qemu/22881

What I like in your patch:
- simplicity
What I don't like in your patch:
- memcpy() and memalign()

What I like in my patch:
- no memcpy()
What I don't like in my patch:
- complexity

But if we want too keep simplicity without memcpy(), we could only
de-activate O_DIRECT on pread() or pwrite() :

 fd_arg = fcntl(fd, F_GETFL);
 if ((fd_arg & O_DIRECT) == 0)
     return 0;
 fcntl(fd, F_SETFL, fd_arg & ~O_DIRECT);

And restore it after.

Regards,
Laurent
-- 
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