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Date: Fri, 5 Aug 2011 14:12:48 +0100
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20110805131247.GC6201@redhat.com>
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Subject: Re: [Qemu-devel] Safely reopening image files by stashing fds
Reply-To: "Daniel P. Berrange" <berrange@redhat.com>
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Supriya Kannery <supriyak@linux.vnet.ibm.com>

On Fri, Aug 05, 2011 at 10:27:00AM +0100, Stefan Hajnoczi wrote:
> On Fri, Aug 5, 2011 at 10:04 AM, Paolo Bonzini <pbonzini@redhat.com> wr=
ote:
> > On 08/05/2011 10:40 AM, Stefan Hajnoczi wrote:
> >>
> >> 4. Implement bdrv_reopen()
> >>
> >> First call bdrv_extract_fds() to stash the file descriptors, then cl=
ose
> >> the
> >> block device. =C2=A0Try opening the new image but if that fails, reo=
pen using
> >> the
> >> stashed file descriptors.
> >
> > Why not do the latter unconditionally?
>=20
> Because you cannot change O_DIRECT on an open fd :(.  This is why
> we're going through this pain.

Hmm, I remember hearing that before, but looking at the current fcntl()
manpage, it claims you *can* change O_DIRECT using SET_FL. Perhaps this
is a newish feature, but it'd be nicer to use it if possible ?

  [man 2 fcntl]
       F_SETFL (long)
              Set  the  file status flags to the value specified by arg. =
 File
              access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation =
flags
              (i.e.,  O_CREAT,  O_EXCL, O_NOCTTY, O_TRUNC) in arg are ign=
ored.
              On Linux this command can only  change  the  O_APPEND,  O_A=
SYNC,
              O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
  [/man]

Regards,
Daniel
--=20
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