Re: [Qemu-devel] [PATCH 3/4] nbd/client: Support requests of additional block sizing info

[Eric Blake]

On 05/22/2018 10:33 AM, Vladimir Sementsov-Ogievskiy wrote:
> 02.05.2018 00:13, Eric Blake wrote:
> > The NBD spec is clarifying [1] that a server may want to advertise
> > different limits for READ/WRITE (in our case, 32M) than for
> > TRIM/ZERO (in our case, nearly 4G).  Implement the client
> > side support for these alternate limits, by always requesting
> > the new information (a compliant server must ignore the
> > request if it is unknown), and by validating anything reported
> > by the server before populating the block layer limits.

I still need to do a v2 of this series based on feedback from the NBD 
list, but answering your question in the meantime:


> >       bs->bl.request_alignment = min ? min : BDRV_SECTOR_SIZE;
> > -    bs->bl.max_pdiscard = max;
> > -    bs->bl.max_pwrite_zeroes = max;
> > +    if (s->info.max_trim) {
> > +        bs->bl.max_pdiscard = MIN(s->info.max_trim, 
> > BDRV_REQUEST_MAX_BYTES);
> > +    } else {
> > +        bs->bl.max_pdiscard = max;
> > +    }
> > +    bs->bl.pdiscard_alignment = s->info.min_trim;
> > +    if (s->info.max_zero) {
> > +        bs->bl.max_pwrite_zeroes = MIN(s->info.max_zero,
> > +                                       BDRV_REQUEST_MAX_BYTES);
> > +    } else {
> > +        bs->bl.max_pwrite_zeroes = max;
> > +    }
> > +    bs->bl.pwrite_zeroes_alignment = s->info.min_zero;
> >       bs->bl.max_transfer = max;
>
> Should not max_transfer be updated too? Looks like it limits all 
> requests, is it right?

max_transfer affects read and write requests, but not write_zero 
requests (where max_pwrite_zeroes is used instead) or trim requests 
(where max_pdiscard is used instead).  Which is precisely the semantics 
we want - the NBD extension is mirroring the fact that qemu already has 
a way to track independent limits for trim/zero that can be larger than 
the normal limits for read/write, because trim/zero do not involve a 
transfer of a large data buffer.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org