Re: [Qemu-block] [PATCH v2 2/3] block/backup: avoid copying less than full target clusters

[Fam Zheng]

On Mon, 02/22 17:07, 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 <address@hidden>
> Signed-off-by: John Snow <address@hidden>
> ---
>  block/backup.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index 76addef..a9a4d5c 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -501,6 +501,7 @@ void backup_start(BlockDriverState *bs, BlockDriverState 
> *target,
>                    BlockJobTxn *txn, Error **errp)
>  {
>      int64_t len;
> +    BlockDriverInfo bdi;
>  
>      assert(bs);
>      assert(target);
> @@ -578,7 +579,14 @@ void backup_start(BlockDriverState *bs, BlockDriverState 
> *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. Instead 
> of
> +     * checking for a backing file, we assume that just copying the data in 
> the
> +     * backup loop is comparable to the unreliable COW. */
> +    bdrv_get_info(job->target, &bdi);

bdrv_get_info can fail and bdi fields are uninitialized.  Pleae test the return
value and handle the error.

Fam

> +    job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
> +
>      job->common.len = len;
>      job->common.co = qemu_coroutine_create(backup_run);
>      block_job_txn_add_job(txn, &job->common);
> -- 
> 2.4.3
>