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Re: [Qemu-devel] [4367] Align file accesses with cache=off (Kevin Wolf,


From: Anthony Liguori
Subject: Re: [Qemu-devel] [4367] Align file accesses with cache=off (Kevin Wolf, Laurent Vivier)
Date: Tue, 06 May 2008 17:33:09 -0500
User-agent: Thunderbird 2.0.0.12 (X11/20080227)

Fabrice Bellard wrote:
A note: in order to avoid uncontrolled recursions, it is better to call
the read/write AIO callback outside the aio_read/write (see
bdrv_aio_read_em).

Personally I would not trust the OS to correctly handle the mix of
O_DIRECT and buffered operations, especially if the corresponding file
regions intersect !

Indeed, why not just allocate a temporary buffer and realign the access such that it's correct? If that involves doing a read first then a write, so be it.

Messing with file flags is very dangerous as there may be a request in-flight already in a different thread. I don't think this patch is safe at all.

Regards,

Anthony Liguori

Fabrice.

Blue Swirl wrote:
Revision: 4367
          http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=4367
Author:   blueswir1
Date:     2008-05-06 17:26:59 +0000 (Tue, 06 May 2008)

Log Message:
-----------
Align file accesses with cache=off (Kevin Wolf, Laurent Vivier)

Modified Paths:
--------------
    trunk/block-raw-posix.c
[...]







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