On 19.12.2016 23:38, sochin jiang wrote:
> Mirroring using 'top' mode without backing file specified on the target can
> be success,
> but end with a disaster.
>
> For example:
> Migration can be success in this situation while the virtual machine on
> the destination
> is no longer usable because of backing lost.
>
> Remind the user earlier and return error in case of misoperation.
>
> Signed-off-by: sochin jiang <sochin.ji...@huawei.com>
> ---
> block/mirror.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/block/mirror.c b/block/mirror.c
> index 301ba92..3476696 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -1038,6 +1038,12 @@ void mirror_start(const char *job_id, BlockDriverState
> *bs,
> error_setg(errp, "Sync mode 'incremental' not supported");
> return;
> }
> + if (mode == MIRROR_SYNC_MODE_TOP && !backing_bs(target))
> + {Syntactic issue: The opening brace should be on the same line as the "if". > + error_setg(errp, "Target Backing required using Sync mode 'top'"); > + return; > + } > + > is_none_mode = mode == MIRROR_SYNC_MODE_NONE; > base = mode == MIRROR_SYNC_MODE_TOP ? backing_bs(bs) : NULL; > mirror_start_job(job_id, bs, BLOCK_JOB_DEFAULT, target, replaces, General issue: For blockdev-mirror, I think this is a legitimate use case. As far as I'm aware, libvirt uses the mirror block job for all backups -- they do so by cancelling the block job after the BLOCK_JOB_READY event instead of letting it complete. So a user might want to mirror some drive somewhere else (in sync=top mode, with the new location not yet having assigned a backing file to it), then cancel the block job after BLOCK_JOB_READY and only assign the backing file at some later point. An even greater issue is that qmp_drive_mirror() opens the target BDS with BDRV_O_NO_BACKING. Therefore, this will always error out with drive-mirror and sync=top (unless the source image does not have a backing file, in which case the sync=top will silently be converted to sync=full). For drive-mirror, the target's backing chain will not be set up until mirror_complete(). Max
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