Re: [Qemu-devel] [Qemu-block] [PATCH v3] drive-mirror: add incremental mode

[John Snow <jsnow@redhat.com>]

On 1/31/19 7:17 AM, mahaocong wrote:
> From: mahaocong <mahaocong@didichuxing.com>
> 

Please use "git send-email" where possible, it will help keep the
formatting right. Use "RESEND" or "v4" when sending out a cover letter
revision, so the tools can see it as a new thread properly.

Sorry for the hassle!

> This patch adds possibility to start mirroring with user-created-bitmap.
> On full mode, mirror create a non-named-bitmap by scanning whole block-chain,
> and on top mode, mirror create a bitmap by scanning the top block layer. So I
> think I can copy a user-created-bitmap and use it as the initial state of the
> mirror, the same as incremental mode drive-backup, and I call this new mode
> as incremental mode drive-mirror.
> 

It makes sense by extension, yes.

> A possible usage scene of incremental mode mirror is live migration. For maintain
> the block data and recover after a malfunction, someone may backup data to ceph
> or other distributed storage. On qemu incremental backup, we need to create a new
> bitmap and attach to block device before the first backup job. Then the bitmap
> records the change after the backup job. If we want to migration this vm, we can
> migrate block data between source and destionation by using drive-mirror directly,
> or use backup data and backup-bitmap to reduce the data should be synchronize.
> To do this, we should first create a new image in destination and set backing file
> as backup image, then set backup-bitmap as the initial state of incremental mode
> drive-mirror, and synchronize dirty block starting with the state set by the last
> incremental backup job. When the mirror complete, we get an active layer on destination,
> and its backing file is backup image on ceph. Then we can do live copy data from
> backing files into overlay images by using block-stream, or do backup continually.
> 
> In this scene, It seems that If the guest os doesn't write too many data after the
> last backup, the incremental mode may transmit less data than full mode or top
> mode. However, if the write data is too many, there is no advantage on incremental
> mode compare with other mode.
> 

It does seem very situational. I suppose there's no real harm in
allowing people to try it, though.

> This scene can be described as following steps:
> 1.create rbd image in ceph, and map nbd device with rbd image.
> 2.create a new bitmap and attach to block device.
> 3.do full mode backup on nbd device and thus sync it to the rbd image.
> 4.severl times incremental mode backup.
> 5.create new image in destination and set its backing file as backup image.
> 6.do live-migration, and migrate block data by incremental mode drive-mirror
>   with bitmap created from step 2.
> 
> Signed-off-by: Ma Haocong <mahaocong@didichuxing.com>
> ---
>  compare with old version, there are following changes:
>  1.replace the copy bitmap function by bdrv_merge_dirty_bitmap
>  2.remove checking for cancelled after mirror_dirty_init_incremental for bitmap
>    copyimg don't have yield point.
>  3.adjuest input parameters on mirror_start_job and mirror_start, and so It is
>    no need to find bitmap on mirror_dirty_init_incremental again.
>  4.assert the bitmap name is NULL on blockdev_mirror_common.
>  5.change the parameter's name in qmp command 'drive-mirror' from 'bitmap_name'
>    to 'bitmap'.
>  6.change the discribe of parameter 'bitmap' in block-core.json.
> 
>  block/mirror.c            | 46 ++++++++++++++++++++++++++++++++++------------
>  blockdev.c                | 41 ++++++++++++++++++++++++++++++++++++-----
>  include/block/block_int.h |  3 ++-
>  qapi/block-core.json      |  7 ++++++-
>  4 files changed, 78 insertions(+), 19 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index ab59ad77e8..c59aefe9f0 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -50,6 +50,7 @@ typedef struct MirrorBlockJob {
>      /* Used to block operations on the drive-mirror-replace target */
>      Error *replace_blocker;
>      bool is_none_mode;
> +    BdrvDirtyBitmap *src_bitmap;
>      BlockMirrorBackingMode backing_mode;
>      MirrorCopyMode copy_mode;
>      BlockdevOnError on_source_error, on_target_error;
> @@ -814,6 +815,15 @@ static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
>      return 0;
>  }
>  
> +/*
> + * init dirty bitmap by using user bitmap. usr->hbitmap will be copy to
> + * mirror bitmap->hbitmap instead of reuse it.
> + */
> +static void coroutine_fn mirror_dirty_init_incremental(MirrorBlockJob *s, Error **errp)
> +{
> +    bdrv_merge_dirty_bitmap(s->dirty_bitmap, s->src_bitmap, NULL, errp);
> +}
> +
>  /* Called when going out of the streaming phase to flush the bulk of the
>   * data to the medium, or just before completing.
>   */
> @@ -839,6 +849,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
>      char backing_filename[2]; /* we only need 2 characters because we are only
>                                   checking for a NULL string */
>      int ret = 0;
> +    Error *local_err = NULL;
>  
>      if (job_is_cancelled(&s->common.job)) {
>          goto immediate_exit;
> @@ -913,9 +924,19 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
>  
>      s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      if (!s->is_none_mode) {
> -        ret = mirror_dirty_init(s);
> -        if (ret < 0 || job_is_cancelled(&s->common.job)) {
> -            goto immediate_exit;
> +        /* incremental mode */
> +        if (s->src_bitmap) {
> +            mirror_dirty_init_incremental(s, &local_err);
> +            if (local_err) {
> +                error_propagate(errp, local_err);
> +                ret = -1;
> +                goto immediate_exit;
> +            }
> +        } else {
> +            ret = mirror_dirty_init(s);
> +            if (ret < 0 || job_is_cancelled(&s->common.job)) {
> +                goto immediate_exit;
> +            }
>          }
>      }
>  
> @@ -1484,7 +1505,8 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
>                               BlockCompletionFunc *cb,
>                               void *opaque,
>                               const BlockJobDriver *driver,
> -                             bool is_none_mode, BlockDriverState *base,
> +                             bool is_none_mode, BdrvDirtyBitmap *src_bitmap,
> +                             BlockDriverState *base,
>                               bool auto_complete, const char *filter_node_name,
>                               bool is_mirror, MirrorCopyMode copy_mode,
>                               Error **errp)
> @@ -1598,6 +1620,7 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
>      s->on_source_error = on_source_error;
>      s->on_target_error = on_target_error;
>      s->is_none_mode = is_none_mode;
> +    s->src_bitmap = src_bitmap;
>      s->backing_mode = backing_mode;
>      s->copy_mode = copy_mode;
>      s->base = base;
> @@ -1664,7 +1687,8 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
>                    BlockDriverState *target, const char *replaces,
>                    int creation_flags, int64_t speed,
>                    uint32_t granularity, int64_t buf_size,
> -                  MirrorSyncMode mode, BlockMirrorBackingMode backing_mode,
> +                  MirrorSyncMode mode, BdrvDirtyBitmap *src_bitmap,
> +                  BlockMirrorBackingMode backing_mode,
>                    BlockdevOnError on_source_error,
>                    BlockdevOnError on_target_error,
>                    bool unmap, const char *filter_node_name,
> @@ -1673,17 +1697,14 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
>      bool is_none_mode;
>      BlockDriverState *base;
>  
> -    if (mode == MIRROR_SYNC_MODE_INCREMENTAL) {
> -        error_setg(errp, "Sync mode 'incremental' not supported");
> -        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, creation_flags, target, replaces,
>                       speed, granularity, buf_size, backing_mode,
>                       on_source_error, on_target_error, unmap, NULL, NULL,
> -                     &mirror_job_driver, is_none_mode, base, false,
> -                     filter_node_name, true, copy_mode, errp);
> +                     &mirror_job_driver, is_none_mode,
> +                     src_bitmap, base, false, filter_node_name, true,
> +                     copy_mode, errp);
>  }
>  
>  void commit_active_start(const char *job_id, BlockDriverState *bs,
> @@ -1707,7 +1728,8 @@ void commit_active_start(const char *job_id, BlockDriverState *bs,
>      mirror_start_job(job_id, bs, creation_flags, base, NULL, speed, 0, 0,
>                       MIRROR_LEAVE_BACKING_CHAIN,
>                       on_error, on_error, true, cb, opaque,
> -                     &commit_active_job_driver, false, base, auto_complete,
> +                     &commit_active_job_driver, false,
> +                     NULL, base, auto_complete,
>                       filter_node_name, false, MIRROR_COPY_MODE_BACKGROUND,
>                       &local_err);
>      if (local_err) {
> diff --git a/blockdev.c b/blockdev.c
> index e6c8349409..969362d36a 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -992,9 +992,7 @@ DriveInfo *drive_new(QemuOpts *all_opts, BlockInterfaceType block_default_type,
>      blk = blockdev_init(filename, bs_opts, &local_err);
>      bs_opts = NULL;
>      if (!blk) {
> -        if (local_err) {
> -            error_propagate(errp, local_err);
> -        }
> +        error_propagate(errp, local_err);
>          goto fail;
>      } else {
>          assert(!local_err);

Why is this here?

> @@ -3663,6 +3661,8 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>                                     BlockDriverState *target,
>                                     bool has_replaces, const char *replaces,
>                                     enum MirrorSyncMode sync,
> +                                   bool has_bitmap,
> +                                   const char *bitmap_name,
>                                     BlockMirrorBackingMode backing_mode,
>                                     bool has_speed, int64_t speed,
>                                     bool has_granularity, uint32_t granularity,
> @@ -3680,6 +3680,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>                                     Error **errp)
>  {
>      int job_flags = JOB_DEFAULT;
> +    BdrvDirtyBitmap *src_bitmap = NULL;
>  
>      if (!has_speed) {
>          speed = 0;
> @@ -3702,6 +3703,24 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>      if (!has_filter_node_name) {
>          filter_node_name = NULL;
>      }
> +    if (!has_bitmap) {
> +        bitmap_name = NULL;
> +    }
> +    /*
> +     * In incremental mode, we should create null name bitmap by
> +     * using user bitmap's granularity.
> +     */
> +    if (sync == MIRROR_SYNC_MODE_INCREMENTAL) {
> +        assert(bitmap_name);
> +        src_bitmap = bdrv_find_dirty_bitmap(bs, bitmap_name);
> +        if (!src_bitmap) {
> +            error_setg(errp, "Error: can't find dirty bitmap "
> +                       "before start incremental drive-mirror");
> +            return;
> +        }
> +        granularity = bdrv_dirty_bitmap_granularity(src_bitmap);
> +    }
> +
>      if (!has_copy_mode) {
>          copy_mode = MIRROR_COPY_MODE_BACKGROUND;
>      }
> @@ -3739,7 +3758,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>       */
>      mirror_start(job_id, bs, target,
>                   has_replaces ? replaces : NULL, job_flags,
> -                 speed, granularity, buf_size, sync, backing_mode,
> +                 speed, granularity, buf_size, sync, src_bitmap, backing_mode,
>                   on_source_error, on_target_error, unmap, filter_node_name,
>                   copy_mode, errp);
>  }
> @@ -3786,6 +3805,16 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>      if (arg->sync == MIRROR_SYNC_MODE_NONE) {
>          source = bs;
>      }
> +    if ((arg->sync == MIRROR_SYNC_MODE_INCREMENTAL) &&
> +        (!arg->has_bitmap)) {
> +        error_setg(errp, "incremental mode must specify the bitmap name");
> +        goto out;
> +    }
> +    if ((arg->sync == MIRROR_SYNC_MODE_INCREMENTAL) &&
> +        (arg->has_granularity)) {
> +        error_setg(errp, "incremental mode can not set bitmap granularity");
> +        goto out;
> +    }

You also want to check if INCREMENTAL isn't set and a bitmap is
provided, that should also error out. See backup.c lines 619..636.

>  
>      size = bdrv_getlength(bs);
>      if (size < 0) {
> @@ -3880,6 +3909,7 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>  
>      blockdev_mirror_common(arg->has_job_id ? arg->job_id : NULL, bs, target_bs,
>                             arg->has_replaces, arg->replaces, arg->sync,
> +                           arg->has_bitmap, arg->bitmap,
>                             backing_mode, arg->has_speed, arg->speed,
>                             arg->has_granularity, arg->granularity,
>                             arg->has_buf_size, arg->buf_size,
> @@ -3937,7 +3967,8 @@ void qmp_blockdev_mirror(bool has_job_id, const char *job_id,
>      bdrv_set_aio_context(target_bs, aio_context);
>  
>      blockdev_mirror_common(has_job_id ? job_id : NULL, bs, target_bs,
> -                           has_replaces, replaces, sync, backing_mode,
> +                           has_replaces, replaces, sync, false, NULL,
> +                           backing_mode,

Why does it make sense to expose this for drive-mirror but not for
blockdev-mirror?

>                             has_speed, speed,
>                             has_granularity, granularity,
>                             has_buf_size, buf_size,
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index f605622216..57a441f992 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -1054,7 +1054,8 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
>                    BlockDriverState *target, const char *replaces,
>                    int creation_flags, int64_t speed,
>                    uint32_t granularity, int64_t buf_size,
> -                  MirrorSyncMode mode, BlockMirrorBackingMode backing_mode,
> +                  MirrorSyncMode mode, BdrvDirtyBitmap *src_bitmap,
> +                  BlockMirrorBackingMode backing_mode,
>                    BlockdevOnError on_source_error,
>                    BlockdevOnError on_target_error,
>                    bool unmap, const char *filter_node_name,
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 762000f31f..e4004cf28b 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1727,6 +1727,11 @@
>  #        (all the disk, only the sectors allocated in the topmost image, or
>  #        only new I/O).
>  #
> +# @bitmap: the name of a pre-existing bitmap to use in incremental mode;
> +#          must be present for incremental mode and absent otherwise. In
> +#          incremental mode, granularity should not be set, because the bitmap's
> +#          granularity is used instead (since 4.0).
> +#

You could possibly say:

"The name of a bitmap to use in incremental mode. This argument must be
present for incremental mode and absent otherwise. In incremental mode,
granularity must not be set, the bitmap's granularity is used instead.
(since 4.0)

- Pre-existing bitmap is the kind usually inferred by parameters named
@bitmap, so that's redundant.
- We enforce no granularity, so use a stronger phrasing "must not"
instead of "should not".


>  # @granularity: granularity of the dirty bitmap, default is 64K
>  #               if the image format doesn't have clusters, 4K if the clusters
>  #               are smaller than that, else the cluster size.  Must be a
> @@ -1768,7 +1773,7 @@
>  { 'struct': 'DriveMirror',
>    'data': { '*job-id': 'str', 'device': 'str', 'target': 'str',
>              '*format': 'str', '*node-name': 'str', '*replaces': 'str',
> -            'sync': 'MirrorSyncMode', '*mode': 'NewImageMode',
> +            'sync': 'MirrorSyncMode', '*bitmap': 'str', '*mode': 'NewImageMode',
>              '*speed': 'int', '*granularity': 'uint32',
>              '*buf-size': 'int', '*on-source-error': 'BlockdevOnError',
>              '*on-target-error': 'BlockdevOnError',
> 

As a general observation, drive_backup and blockdev_backup both do their
error checking in backup_job_create, which lets drive-backup and
blockdev-backup share error checking for incremental mode.

This patch adds seven usages of the word "incremental" to blockdev.c,
where no reference existed before.

Maybe the changes should be localized to mirror.c more extensively?

...hmm, it looks like mirror takes basically the opposite approach and
does almost no checking in mirror_start or mirror_start_job. ...I guess
it's fine to do it in blockdev.c to follow what mirror is already doing.
Nevermind!