Re: [PATCH v4 2/4] block/nbd: define new max_write_zero_fast limit

[Vladimir Sementsov-Ogievskiy]

23.07.2020 23:32, Eric Blake wrote:
> On 6/11/20 11:26 AM, Vladimir Sementsov-Ogievskiy wrote:
> > The NBD spec was recently updated to clarify that max_block doesn't
> > relate to NBD_CMD_WRITE_ZEROES with NBD_CMD_FLAG_FAST_ZERO (which
> > mirrors Qemu flag BDRV_REQ_NO_FALLBACK).
> >
> > bs->bl.max_write_zero_fast is zero by default which means using
> > max_pwrite_zeroes. Update nbd driver to allow larger requests with
> > BDRV_REQ_NO_FALLBACK.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > ---
> >   block/nbd.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/block/nbd.c b/block/nbd.c
> > index 4ac23c8f62..b0584cf68d 100644
> > --- a/block/nbd.c
> > +++ b/block/nbd.c
> > @@ -1956,6 +1956,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, 
> > Error **errp)
> >       bs->bl.request_alignment = min;
> >       bs->bl.max_pdiscard = QEMU_ALIGN_DOWN(INT_MAX, min);
> > +    bs->bl.max_pwrite_zeroes_fast = bs->bl.max_pdiscard;
> >       bs->bl.max_pwrite_zeroes = max;
>
> Do we even need max_pwrite_zeroes_fast?  Doesn't qemu behave correctly if we 
> just blindly assign max_pdiscard and max_pwrite_zeroes to the same value near 
> 2G?

Without BDRV_REQ_NO_FALLBACK, max_block is an actual limit for WRITE_ZERO.. So, 
in my understanding, it doesn't


--
Best regards,
Vladimir