Re: [Qemu-block] [PATCH v6] migration/block: use blk_pwrite_zeroes for each zero cluster

[858585 jemmy]

On Fri, Apr 14, 2017 at 2:00 PM, Fam Zheng <address@hidden> wrote:
> On Thu, 04/13 10:34, address@hidden wrote:
>> From: Lidong Chen <address@hidden>
>>
>> BLOCK_SIZE is (1 << 20), qcow2 cluster size is 65536 by default,
>> this may cause the qcow2 file size to be bigger after migration.
>> This patch checks each cluster, using blk_pwrite_zeroes for each
>> zero cluster.
>>
>> Reviewed-by: Stefan Hajnoczi <address@hidden>
>> Signed-off-by: Lidong Chen <address@hidden>
>> ---
>> v6 changelog:
>> Fix up some grammar in the comment.
>> ---
>>  migration/block.c | 35 +++++++++++++++++++++++++++++++++--
>>  1 file changed, 33 insertions(+), 2 deletions(-)
>>
>> diff --git a/migration/block.c b/migration/block.c
>> index 7734ff7..41c7a55 100644
>> --- a/migration/block.c
>> +++ b/migration/block.c
>> @@ -885,6 +885,8 @@ static int block_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>      int64_t total_sectors = 0;
>>      int nr_sectors;
>>      int ret;
>> +    BlockDriverInfo bdi;
>> +    int cluster_size;
>>
>>      do {
>>          addr = qemu_get_be64(f);
>> @@ -919,6 +921,15 @@ static int block_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>                      error_report_err(local_err);
>>                      return -EINVAL;
>>                  }
>> +
>> +                ret = bdrv_get_info(blk_bs(blk), &bdi);
>> +                if (ret == 0 && bdi.cluster_size > 0 &&
>> +                    bdi.cluster_size <= BLOCK_SIZE &&
>> +                    BLOCK_SIZE % bdi.cluster_size == 0) {
>> +                    cluster_size = bdi.cluster_size;
>> +                } else {
>> +                    cluster_size = BLOCK_SIZE;
>> +                }
>>              }
>>
>>              if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {
>> @@ -932,10 +943,30 @@ static int block_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>                                          nr_sectors * BDRV_SECTOR_SIZE,
>>                                          BDRV_REQ_MAY_UNMAP);
>>              } else {
>> +                int i;
>> +                int64_t cur_addr;
>> +                uint8_t *cur_buf;
>> +
>>                  buf = g_malloc(BLOCK_SIZE);
>>                  qemu_get_buffer(f, buf, BLOCK_SIZE);
>> -                ret = blk_pwrite(blk, addr * BDRV_SECTOR_SIZE, buf,
>> -                                 nr_sectors * BDRV_SECTOR_SIZE, 0);
>> +                for (i = 0; i < BLOCK_SIZE / cluster_size; i++) {
>> +                    cur_addr = addr * BDRV_SECTOR_SIZE + i * cluster_size;
>> +                    cur_buf = buf + i * cluster_size;
>> +
>> +                    if ((!block_mig_state.zero_blocks ||
>> +                        cluster_size < BLOCK_SIZE) &&
>> +                        buffer_is_zero(cur_buf, cluster_size)) {
>> +                        ret = blk_pwrite_zeroes(blk, cur_addr,
>> +                                                cluster_size,
>> +                                                BDRV_REQ_MAY_UNMAP);
>> +                    } else {
>> +                        ret = blk_pwrite(blk, cur_addr, cur_buf,
>> +                                         cluster_size, 0);
>> +                    }
>> +                    if (ret < 0) {
>> +                        break;
>> +                    }
>> +                }
>>                  g_free(buf);
>>              }
>
> Sorry for asking this question so late, but, before it gets too late: did you
> evaluate the performance impact of this change under real world workload?
>
> Effectively, if no cluster is zero, this patch still splits a big write into
> small ones, which is the opposition of usual performance optimizations (i.e.
> trying to coalesce requests).

I test this patch for qcow2, the migration speed is the same before
apply this patch.

Do you know some other format which have very small cluster size?

>
> Fam