Re: [Qemu-devel] [PATCH 1/3] block/backup: deal with zero detection

[John Snow <jsnow@redhat.com>]

On 7/30/19 12:32 PM, Vladimir Sementsov-Ogievskiy wrote:
> We have detect_zeroes option, so at least for blockdev-backup user
> should define it if zero-detection is needed. For drive-backup leave
> detection enabled by default but do it through existing option instead
> of open-coding.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/backup.c | 15 ++++++---------
>  blockdev.c     |  8 ++++----
>  2 files changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index 715e1d3be8..f4aaf08df3 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -110,7 +110,10 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
>      BlockBackend *blk = job->common.blk;
>      int nbytes;
>      int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
> -    int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
> +    int write_flags =
> +            (job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0) |
> +            (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
> +
>  
>      assert(QEMU_IS_ALIGNED(start, job->cluster_size));
>      hbitmap_reset(job->copy_bitmap, start, job->cluster_size);
> @@ -128,14 +131,8 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
>          goto fail;
>      }
>  
> -    if (buffer_is_zero(*bounce_buffer, nbytes)) {
> -        ret = blk_co_pwrite_zeroes(job->target, start,
> -                                   nbytes, write_flags | BDRV_REQ_MAY_UNMAP);
> -    } else {
> -        ret = blk_co_pwrite(job->target, start,
> -                            nbytes, *bounce_buffer, write_flags |
> -                            (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
> -    }
> +    ret = blk_co_pwrite(job->target, start, nbytes, *bounce_buffer,
> +                        write_flags);
>      if (ret < 0) {
>          trace_backup_do_cow_write_fail(job, start, ret);
>          if (error_is_read) {
> diff --git a/blockdev.c b/blockdev.c
> index 4d141e9a1f..a94d754504 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -3434,7 +3434,7 @@ static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>      BlockJob *job = NULL;
>      BdrvDirtyBitmap *bmap = NULL;
>      AioContext *aio_context;
> -    QDict *options = NULL;
> +    QDict *options;
>      Error *local_err = NULL;
>      int flags, job_flags = JOB_DEFAULT;
>      int64_t size;
> @@ -3529,10 +3529,10 @@ static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>          goto out;
>      }
>  
> +    options = qdict_new();
> +    qdict_put_str(options, "discard", "unmap");
> +    qdict_put_str(options, "detect-zeroes", "unmap");
>      if (backup->format) {
> -        if (!options) {
> -            options = qdict_new();
> -        }
>          qdict_put_str(options, "driver", backup->format);
>      }
>  
> 

I'm less sure of this one personally. Is it right to always try to set
unmap on the target?

I like the idea of removing special cases and handling things more
centrally though, but I'll want Max (or Kevin) to take a peek.

--js