Re: [Qemu-devel] [PATCH 1/1] nbd: implement bdrv_get_info callback

[Eric Blake <eblake@redhat.com>]

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On 01/26/2018 06:39 AM, Edgar Kaziakhmedov wrote:
> PIng
> 
> So, let me know if I need to make any changes in patch
> 
> On 1/18/18 1:09 PM, Paolo Bonzini wrote:
>> On 18/01/2018 12:51, Edgar Kaziakhmedov wrote:
>>> +static int nbd_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
>>> +{
>>> +    if (bs->supported_zero_flags & BDRV_REQ_MAY_UNMAP) {
>>> +        bdi->can_write_zeroes_with_unmap = true;
>>> +    }
>>> +    return 0;
>>> +}
>>> +
>> Other drivers set the flag always, while NBD only sets it if the server
>> knows the flag.

Well, other drivers may be able to always implement it (NBD can only
implement it if the server supports WRITE_ZEROES - and I'm even in the
middle of working up an nbdkit patch [1] that makes it easier to write
an NBD server that specifically does not support WRITE_ZEROES to make
code paths like this easier to test)

[1]

>>
>> I think NBD is more correct, so:
>>
>> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Agreed; I'm fine queueing this on my NBD queue, except I'd first like to
hear Kevin's opinion:

>>
>> However, it would be nice to remove can_write_zeroes_with_unmap from
>> BlockDriverInfo, and make bdrv_can_write_zeroes_with_unmap just return
>> !!(bs->supported_zero_flags & BDRV_REQ_MAY_UNMAP).  Kevin, what do you
>> think?

Actually, I may even just give a shot at writing this alternative patch,
to make Kevin's decision easier.

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


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