Re: [Qemu-devel] [PATCH 6/8] QAPI: add command for live block commit, 'block-commit'

[Eric Blake]

On 09/14/2012 07:41 AM, Jeff Cody wrote:
> The command for live block commit is added, which has the following
> arguments:
> 
> device: the block device to perform the commit on (mandatory)
> base:   the base image to commit into; optional (if not specified,
>         it is the underlying original image)
> top:    the top image of the commit - all data from inside top down
>         to base will be committed into base. optional (if not specified,
>         it is the active image) - see note below
> speed:  maximum speed, in bytes/sec
> on_error: action to take on error (optional - default is report)

Shouldn't this be on-error, with a dash?  Also, this doesn't match the
actual commit, since you pulled it while waiting to rebase on Paolo's
patches.

> 
> note: eventually this will support merging down the active layer,
>       but that code is not yet complete.  If the active layer is passed
>       in currently as top, or top is left to the default, then the error
>       QERR_TOP_NOT_FOUND will be returned.

I don't think we need a new error category for this particular failure
(and in 4/8, you labeled the error as generic); so it is sufficient to
state that 'an error will be returned'.

> 
> The is done as a block job, so upon completion a BLOCK_JOB_COMPLETED will
> be emitted.
> 

> +++ b/blockdev.c
> @@ -825,7 +825,6 @@ exit:
>      return;
>  }
>  
> -
>  static void eject_device(BlockDriverState *bs, int force, Error **errp)

Spurious whitespace change.


> +void qmp_block_commit(const char *device,
> +                      bool has_base, const char *base,
> +                      bool has_top, const char *top,
> +                      bool has_speed, int64_t speed,
> +                      Error **errp)
> +{
> +    BlockDriverState *bs;
> +    BlockDriverState *base_bs, *top_bs;
> +    Error *local_err = NULL;
> +    /* This will be part of the QMP command, if/when the
> +     * BlockdevOnError change for blkmirror makes it in
> +     */
> +    BlockErrorAction on_error = BLOCK_ERR_REPORT;
> +
> +    /* drain all i/o before commits */
> +    bdrv_drain_all();

Is this technically necessary for now?  Since you are forbidding actions
on the active image for now, and changing the active image (via snapshot
or pull) already drains all I/O, there should be nothing remaining to
drain for any of the backing files.  Obviously, it will be necessary in
the future when you do add support for committing the active layer.

> +++ b/qapi-schema.json
> @@ -1404,6 +1404,41 @@
>    'returns': 'str' }
>  
>  ##
> +# @block-commit
> +#
> +# Live commit of data from child image nodes into parent nodes - i.e.,
> +# writes data between 'top' and 'base' into 'base'.
> +#
> +# @device:  the name of the device
> +#
> +# @base:   #optional The file name of the parent image of the device to write
> +#                    data into.  If not specified, this is the original 
> parent
> +#                    image.

I though we wanted to fix the terminology to avoid 'parent'; how about:

The file name of the backing image to write data into.  If not
specified, this is the deepest backing image.

> +#
> +# @top:    #optional The file name of the child image, above which data will
> +#                    not be committed down.  If not specified, this is one
> +#                    layer below the active layer (i.e. active->backing_hd).

Again, how about:

The file name of the backing image within the chain which contains the
data to be committed down.  If not specified...

> +#
> +#                    If top == base, that is an error.
> +#
> +#
> +# @speed:  #optional the maximum speed, in bytes per second
> +#
> +# Returns: Nothing on success
> +#          If commit or stream is already active on this device, DeviceInUse
> +#          If @device does not exist, DeviceNotFound
> +#          If image commit is not supported by this device, NotSupported
> +#          If @base does not exist, BaseNotFound
> +#          If @top does not exist, TopNotFound

BaseNotFound is a generic error, and I'm arguing that TopNotFound should
be likewise.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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