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

[Stefan Hajnoczi]

On Wed, Apr 12, 2017 at 09:51:23AM +0800, 858585 jemmy wrote:
> On Wed, Apr 12, 2017 at 9:27 AM, 858585 jemmy <address@hidden> wrote:
> > 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.
> 
> Sorry,  you are right.
> it will cause the destination QEMU process scanning for zeroes again.
> 
> How about this?
> 
>                 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;
>                     }
>                 }

Thanks, looks good to me.  Please send a new revision of the patch.

You can already add my:

Reviewed-by: Stefan Hajnoczi <address@hidden>

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