Re: [Qemu-devel] [V5 Patch 3/4]Qemu: Command "block_set" for dynamic block params change

[Anthony Liguori]

On 07/27/2011 08:43 AM, Michael Tokarev wrote:
> > Index: qemu/block.c
> > ===================================================================
> > --- qemu.orig/block.c
> > +++ qemu/block.c
> > @@ -651,6 +651,34 @@ unlink_and_fail:
> >       return ret;
> >   }
> >
> > +int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
> > +{
> > +    BlockDriver *drv = bs->drv;
> > +    int ret = 0, open_flags;
> > +
> > +    /* Quiesce IO for the given block device */
> > +    qemu_aio_flush();
> > +    if (bdrv_flush(bs)) {
> > +        qerror_report(QERR_DATA_SYNC_FAILED, bs->device_name);
> > +        return ret;
> > +    }
> > +    open_flags = bs->open_flags;
> > +    bdrv_close(bs);
> > +
> > +    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
> > +    if (ret<  0) {
> > +        /* Reopen failed. Try to open with original flags */
> > +        qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
> > +        ret = bdrv_open(bs, bs->filename, open_flags, drv);
> > +        if (ret<  0) {
> > +            /* Reopen failed with orig and modified flags */
> > +            abort();
> > +        }
>
> Can we please avoid this stuff completely?  Just keep the
> old device open still, until you're sure new one is ok.

I may be misremembering, but I thought Christoph had mentioned wanting 
to write a kernel patch to toggle O_DIRECT through fcntl().

This seems like the only way to make this not racy to me.

Regards,

Anthony Liguori