Re: [Qemu-devel] [PATCH 1/5] block: split write_zeroes always

[Eric Blake <eblake@redhat.com>]

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On 05/17/2016 10:34 AM, Kevin Wolf wrote:
> Am 17.05.2016 um 11:15 hat Denis V. Lunev geschrieben:
>> We should split requests even if they are less than write_zeroes_alignment.
>> For example we can have the following request:
>>   offset 62k
>>   size   4k
>>   write_zeroes_alignment 64k
>> The original code sent 1 request covering 2 qcow2 clusters, and resulted
>> in both clusters being allocated. But by splitting the request, we can
>> cater to the case where one of the two clusters can be zeroed as a
>> whole, for only 1 cluster allocated after the operation.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Eric Blake <eblake@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> ---
>>  block/io.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/io.c b/block/io.c
>> index cd6d71a..6a24ea8 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -1172,13 +1172,13 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>>          /* Align request.  Block drivers can expect the "bulk" of the request
>>           * to be aligned.
>>           */
>> -        if (bs->bl.write_zeroes_alignment
>> -            && num > bs->bl.write_zeroes_alignment) {
>> +        if (bs->bl.write_zeroes_alignment) {
>>              if (sector_num % bs->bl.write_zeroes_alignment != 0) {
>>                  /* Make a small request up to the first aligned sector.  */
>>                  num = bs->bl.write_zeroes_alignment;
>>                  num -= sector_num % bs->bl.write_zeroes_alignment;
> 
> Turns out this doesn't work. If this is a small request that zeros
> something in the middle of a single cluster (i.e. we have untouched data
> both before and after the request in the same cluster), then num can now
> become greater than nb_sectors, so that we end up zeroing too much.

I'm planning on folding in a working version of this patch in my
byte-based write_zeroes conversion series.  As part of the patch, I'm
also hoisting the division out of the loop (no guarantees that the
compiler can spot that bs->bl.write_zeroes_alignment will be a power of
two, to optimize it to a shift).



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


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