Re: [Qemu-devel] [PATCH v3 16/22] block: Wording tweaks to write zeroes limits

[Eric Blake <eblake@redhat.com>]

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On 06/24/2016 12:12 AM, Fam Zheng wrote:
> On Thu, 06/23 16:37, Eric Blake wrote:
>> Improve the documentation of the write zeroes limits, to mention
>> additional constraints that drivers should observe.  Worth squashing
>> into commit cf081fca, if that hadn't been pushed already :)
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> ---
>> v3: new patch, split off from "block: Switch discard length bounds..."
>> ---
>>  include/block/block_int.h | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 7d2b152..7a4a00f 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -331,11 +331,14 @@ typedef struct BlockLimits {
>>      int64_t discard_alignment;
>>
>>      /* maximum number of bytes that can zeroized at once (since it is
>> -     * signed, it must be < 2G, if set) */
>> +     * signed, it must be < 2G, if set), should be multiple of
>> +     * pwrite_zeroes_alignment. May be 0 if no inherent 32-bit limit */
> 
> "inherent 32-bit limit"? What is special about 32-bit other than this field is
> 32-bit? Anyway,
> 
> Reviewed-by: Fam Zheng <famz@redhat.com>

Someday, we may want to bump it up to a 64-bit field, and convert the
driver callback for write_zeroes to take a 64-bit size.  But first we'd
have to audit to make sure all drivers properly advertise their maximum
(for example, NBD is still hard-coded to a 32-bit maximum write_zeroes,
even if the callback would permit a 64-bit size).

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


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