Re: [Qemu-devel] [PATCH v2 5/9] block: Pass unaligned discard requests to drivers

[Eric Blake]

On 11/17/2016 04:26 PM, Max Reitz wrote:
> On 17.11.2016 21:13, Eric Blake wrote:
>> Discard is advisory, so rounding the requests to alignment
>> boundaries is never semantically wrong from the data that
>> the guest sees.  But at least the Dell Equallogic iSCSI SANs
>> has an interesting property that its advertised discard
>> alignment is 15M, yet documents that discarding a sequence
>> of 1M slices will eventually result in the 15M page being
>> marked as discarded, and it is possible to observe which
>> pages have been discarded.
>>

>>      max_pdiscard = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_pdiscard, 
>> INT_MAX),
>>                                     align);
>> -    assert(max_pdiscard);
>> +    assert(max_pdiscard >= bs->bl.request_alignment);
>>
>>      while (count > 0) {
>>          int ret;
>> -        int num = MIN(count, max_pdiscard);
>> +        int num = count;
>> +
>> +        if (head) {
>> +            /* Make small requests to get to alignment boundaries. */
>> +            num = MIN(count, align - head);
>> +            if (!QEMU_IS_ALIGNED(num, bs->bl.request_alignment)) {
>> +                num %= bs->bl.request_alignment;
>> +            }
> 
> Could be written as
> 
> num = num % bs->bl.request_alignment ?: num;
> 
> But that's up to you.
> 
> More importantly, is it possible that request_alignment >
> pdiscard_alignment? In that case, align would be request_alignment, head
> would be less than request_alignment but could be more than
> pdiscard_alignment.

pdiscard_alignment can be 0 (no inherent limit); but it if it is
nonzero, it must be at least request_alignment.  The block layer (should
be, if it is not already) enforcing that as part of the
.bdrv_refresh_limits() callback contract; at any rate, it is documented
that way in block_int.h

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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