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

[Eric Blake <eblake@redhat.com>]

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