Re: [Qemu-devel] [PATCH] block: fix big write

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/12/2014 02:41, Ming Lei wrote:
> On Wed, Dec 10, 2014 at 1:45 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 08/12/2014 08:19, Ming Lei wrote:
>>>>>
>>>>> Alternatively, I'd accept a SCSI patch setting max_ws_blocks and friends
>>>>> to 2GB - 1 block.
>>> It should be better to not introduce the limit and split the writes
>>> into size of 2GB - 1 block since there is only the limit for write zero.
>>
>> Why? That's exactly what the max_ws_blocks is for, and there's code in
>> the guest already to do the split.  We're talking about 2GB, not 1MB.
> 
> The split in write same does not cover write zero, and that is the problem.
> Otherwise write same just works fine. That said write same of QEMU SCSI
> can work well without max write same sectors limit.
> 
> If we introduce the limit of max write same sectors, this limit will be put
> on both write zero and write non-zero path.

Yeah, but the 2GB limit happens also for the regular I/O path.  The
quirk is that it doesn't happen for non-zero WRITE SAME, not the other
way round.

> Also "MAXIMUM WRITE SAME LENGTH" is just introduced on sbc3r35
> in Jan, 2013, and some old host drivers may not support it, and not using
> the limit should have better compatibility.

Again, we're talking of 2GB and this is something that should never
happen in practice.

I'll write the patch myself.

Paolo

> Thanks,
> Ming Lei
> 
>