Re: [Qemu-devel] [PATCH v3 2/3] block/backup: avoid copying less than full target clusters

[Fam Zheng <famz@redhat.com>]

On Tue, 02/23 19:17, John Snow wrote:
> During incremental backups, if the target has a cluster size that is
> larger than the backup cluster size and we are backing up to a target
> that cannot (for whichever reason) pull clusters up from a backing image,
> we may inadvertantly create unusable incremental backup images.
> 
> For example:
> 
> If the bitmap tracks changes at a 64KB granularity and we transmit 64KB
> of data at a time but the target uses a 128KB cluster size, it is
> possible that only half of a target cluster will be recognized as dirty
> by the backup block job. When the cluster is allocated on the target
> image but only half populated with data, we lose the ability to
> distinguish between zero padding and uninitialized data.
> 
> This does not happen if the target image has a backing file that points
> to the last known good backup.
> 
> Even if we have a backing file, though, it's likely going to be faster
> to just buffer the redundant data ourselves from the live image than
> fetching it from the backing file, so let's just always round up to the
> target granularity.
> 
> The same logic applies to backup modes top, none, and full. Copying
> fractional clusters without the guarantee of COW is dangerous, but even
> if we can rely on COW, it's likely better to just re-copy the data.
> 
> Reported-by: Fam Zheng <famz@redhat.com>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/backup.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index 76addef..6e9f53d 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -501,6 +501,8 @@ void backup_start(BlockDriverState *bs, BlockDriverState *target,
>                    BlockJobTxn *txn, Error **errp)
>  {
>      int64_t len;
> +    BlockDriverInfo bdi;
> +    int ret;
>  
>      assert(bs);
>      assert(target);
> @@ -570,15 +572,30 @@ void backup_start(BlockDriverState *bs, BlockDriverState *target,
>          goto error;
>      }
>  
> -    bdrv_op_block_all(target, job->common.blocker);
> -
>      job->on_source_error = on_source_error;
>      job->on_target_error = on_target_error;
>      job->target = target;
>      job->sync_mode = sync_mode;
>      job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
>                         sync_bitmap : NULL;
> -    job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
> +
> +    /* If there is no backing file on the target, we cannot rely on COW if our
> +     * backup cluster size is smaller than the target cluster size. Even for
> +     * targets with a backing file, try to avoid COW if possible. */
> +    ret = bdrv_get_info(job->target, &bdi);
> +    if (ret < 0 && !target->backing) {
> +        error_setg_errno(errp, -ret,
> +            "Can't determine cluster size of target that has no backing file. "
> +            "This may create an unusable destination image. Aborting.");

Most error messages don't end with a period, but that's not a hard rule AFAICT.

> +        goto error;
> +    } else if (ret < 0 && target->backing) {
> +        /* Not fatal; just trudge on ahead. */
> +        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
> +    } else {
> +        job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
> +    }
> +
> +    bdrv_op_block_all(target, job->common.blocker);
>      job->common.len = len;
>      job->common.co = qemu_coroutine_create(backup_run);
>      block_job_txn_add_job(txn, &job->common);
> -- 
> 2.4.3
> 

Reviewed-by: Fam Zheng <famz@redhat.com>