Re: [Qemu-devel] Safely reopening image files by stashing fds

[Supriya Kannery]

On 08/05/2011 09:19 PM, Anthony Liguori wrote:
> On 08/05/2011 10:43 AM, Kevin Wolf wrote:
> > Am 05.08.2011 17:24, schrieb Stefan Hajnoczi:
> > > On Fri, Aug 5, 2011 at 3:28 PM, Christoph Hellwig<address@hidden> wrote:
> > > > On Fri, Aug 05, 2011 at 02:12:48PM +0100, Daniel P. Berrange wrote:
> > > > > > 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 ?
> > > >
> > > > It's been there since day 1 of O_DIRECT support.
> > >
> > > Sorry, my bad. So for Linux we could just use fcntl for
> > > block_set_hostcache and not bother with reopening. However, we will
> > > need to reopen should we wish to support changing O_DSYNC.
> >
> > We do wish to support that.
> >
> > Anthony thinks that allowing the guest to toggle WCE is a prerequisite
> > for making cache=writeback the default. And this is something that I
> > definitely want to do for 1.0.
>
> Indeed.

We discussed the following so far...
1. How to safely reopen image files
2. Dynamic hostcache change
3. Support for dynamic change of O_DSYNC

Since 2 is independent of 1, shall I go ahead implementing
hostcache change using fcntl.

Implementation for safely reopening image files using "BDRVReopenState"
can be done separately as a pre-requisite before implementing 3

Thanks, Supriya

> Regards,
>
> Anthony Liguori
>
> > Kevin