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List: qemu-devel
Subject: Re: [Qemu-devel] PATCH: Control over drive open modes for backing file
From: "Daniel P. Berrange" <berrange () redhat ! com>
Date: 2008-07-31 16:10:36
Message-ID: 20080731161036.GL18548 () redhat ! com
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On Thu, Jul 31, 2008 at 05:01:13PM +0100, Jamie Lokier wrote:
> Blue Swirl wrote:
> > On 7/31/08, Daniel P. Berrange <berrange@redhat.com> wrote:
> > > On Thu, Jul 31, 2008 at 02:46:55PM +0100, Paul Brook wrote:
> > > > > +#define BDRV_O_RDONLY 0x0001 /* Force read-only */
> > > > > +#define BDRV_O_WRONLY 0x0002 /* Force writeable, no fallback */
> > > > > +#define BDRV_O_RDWR 0x0003 /* Try writeable, fallback to read-only
> > > > > */
> > > >
> > > > This is IMHO really misleading. Normal O_* are not bitflags. The code uses
> > > > these as bitflags sometimes, which means your descriptions are contradictory.
> > >
> > >
> > > One alternative approach I considered would be to not have an explicit
> > > flag for writable, and instead have a flag to explicitly indicate that
> > > fallback to read-only shouldn't be attempted.
> > >
> > >
> > > #define BDRV_O_RDONLY 0x0001
> > >
> > > #define BDRV_O_NO_RO_FALLBACK 0x0002
> > >
> > > This would probably make the patch smaller because I won't need to update
> > > all the callers which assume flags of '0' gives a writable file, falling
> > > back to RO.
> > >
> > > Other suggestions welcome too...
> >
> > Write-only should mean that only writing is allowed, read access
> > should not be needed.
>
> You can't write to most formats unless you can read the metadata.
> Flat is the exception, but WRONLY doesn't seem particularly useful.
>
> Whereas read-only floppy images, for example, resemble real hardware.
>
> I would suggest: read-only, read-write, and try-write (traditional behaviour).
Functionally, that's what the implementation actually does - its just bad
flag naming that's confusing things here.
Daniel
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