Re: [Qemu-block] [PATCH v4] migration/block: use blk_pwrite_zeroes for each zero cluster

[858585 jemmy]

On Tue, Apr 11, 2017 at 11:59 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Tue, Apr 11, 2017 at 08:05:12PM +0800, address@hidden wrote:
>> From: Lidong Chen <address@hidden>
>>
>> BLOCK_SIZE is (1 << 20), qcow2 cluster size is 65536 by default,
>> this maybe cause the qcow2 file size is bigger after migration.
>> This patch check each cluster, use blk_pwrite_zeroes for each
>> zero cluster.
>>
>> Signed-off-by: Lidong Chen <address@hidden>
>> ---
>>  migration/block.c | 33 +++++++++++++++++++++++++++++++--
>>  1 file changed, 31 insertions(+), 2 deletions(-)
>>
>> diff --git a/migration/block.c b/migration/block.c
>> index 7734ff7..5d0635a 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;
>
> This is a nice trick to unify code paths.  It has a disadvantage though:
>
> If the "zero blocks" migration capability is enabled and the drive has
> no cluster_size (e.g. raw files), then the source QEMU process has
> already scanned for zeroes.  CPU is wasted scanning for zeroes in the
> destination QEMU process.
>
> Given that disk images can be large we should probably avoid unnecessary
> scanning.  This is especially true because there's no other reason
> (besides zero detection) to pollute the CPU cache with data from the
> disk image.
>
> In other words, we should only scan for zeroes when
> !block_mig_state.zero_blocks || cluster_size < BLOCK_SIZE.

This case, the source qemu process will add BLK_MIG_FLAG_ZERO_BLOCK flag.
It will call blk_pwrite_zeroes already before apply this patch.
so destination QEMU process will not scanning for zero cluster.

There are two reason cause the destination QEMU process receive the
block which don't have
BLK_MIG_FLAG_ZERO_BLOCK flag.
1.the source QEMU process is old version, or !block_mig_state.zero_blocks.
2.the content of BLOCK_SIZE is not zero.

So if the destination QEMU process receive the block which don't have
BLK_MIG_FLAG_ZERO_BLOCK flag, it already mee the condition
!block_mig_state.zero_blocks || cluster_size < BLOCK_SIZE.

so i think it's unnecessary to check this condition again.

Thanks.

>
>> +                }
>>              }
>>
>>              if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {
>> @@ -932,10 +943,28 @@ 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 (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);
>>              }