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Subject: Re: [Qemu-devel] [PATCH] Align file accesses with cache=off (O_DIRECT)
From: Laurent Vivier <Laurent.Vivier@bull.net>
In-Reply-To: <4815EE89.70407@suse.de>
References: <4807514B.9040607@suse.de>  <4815EE89.70407@suse.de>
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Date: Tue, 29 Apr 2008 11:01:07 +0200
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To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@suse.de>


Le lundi 28 avril 2008 =C3=A0 17:34 +0200, Kevin Wolf a =C3=A9crit :
> Kevin Wolf schrieb:
> > In December a patch was applied which introduced the cache=3Doff opti=
on to
> > -drive. When using this option files are opened with the O_DIRECT fla=
g.
> > This means that all accesses have to be aligned. The patch made a cou=
ple
> > of changes in this respect, still in other places they are missing (e=
.g.
> > you can't use cache=3Doff with qcow(2) files).
> >=20
> > This patch implements wrappers for raw_pread and raw_pwrite which ali=
gn
> > all file accesses and make qcow(2) work with cache=3Doff. This method
> > might not be the most performant one (compared to fixing qcow, qcow2 =
and
> > everything else that might be using unaligned accesses), but unaligne=
d
> > accesses don't happen that frequently and with this patch really all
> > image accesses should be covered.
> >=20
> > Signed-off-by: Kevin Wolf <kwolf@suse.de>
>=20
> 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 =3D fcntl(fd, F_GETFL);
 if ((fd_arg & O_DIRECT) =3D=3D 0)
     return 0;
 fcntl(fd, F_SETFL, fd_arg & ~O_DIRECT);

And restore it after.

Regards,
Laurent
--=20
------------- Laurent.Vivier@bull.net ---------------
"The best way to predict the future is to invent it."
- Alan Kay