Re: [PATCH qemu 1/4] drive-mirror: add support for sync=bitmap mode=never

[Fabian Grünbichler]

On October 2, 2020 9:06 am, Markus Armbruster wrote:
> Fabian Grünbichler <f.gruenbichler@proxmox.com> writes:
> 
>> From: John Snow <jsnow@redhat.com>
>>
>> This patch adds support for the "BITMAP" sync mode to drive-mirror and
>> blockdev-mirror. It adds support only for the BitmapSyncMode "never,"
>> because it's the simplest mode.
>>
>> This mode simply uses a user-provided bitmap as an initial copy
>> manifest, and then does not clear any bits in the bitmap at the
>> conclusion of the operation.
>>
>> Any new writes dirtied during the operation are copied out, in contrast
>> to backup. Note that whether these writes are reflected in the bitmap
>> at the conclusion of the operation depends on whether that bitmap is
>> actually recording!
>>
>> This patch was originally based on one by Ma Haocong, but it has since
>> been modified pretty heavily.
>>
>> Suggested-by: Ma Haocong <mahaocong@didichuxing.com>
>> Signed-off-by: Ma Haocong <mahaocong@didichuxing.com>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
>> ---
> [...]
>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>> index 2d94873ca0..dac5497084 100644
>> --- a/qapi/block-core.json
>> +++ b/qapi/block-core.json
>> @@ -1961,10 +1961,19 @@
>>  #        (all the disk, only the sectors allocated in the topmost image, or
>>  #        only new I/O).
>>  #
>> +# @bitmap: The name of a bitmap to use for sync=bitmap mode. This argument 
>> must
>> +#          be present for bitmap mode and absent otherwise. The bitmap's
> 
> What is "bitmap mode"?  Do you mean "present when @bitmap-mode is, else
> absent"?

bitmap mode is sync=bitmap , as in the first sentence. if you set 
sync=bitmap, you must specify a bitmap and a bitmap-mode. if you use 
another sync mode, you must not specify a bitmap or a bitmap-mode.

there is also a 'sugar' sync mode 'incremental' that desugars to 
sync=bitmap with bitmap-mode=on-success.  I guess that should also be 
mentioned somewhere in QAPI, it's mainly there since MirrorSyncMode has 
it as possible value, it's semantics are straight-forward to map onto 
this combination, and it's how the sync modes are known from backup 
jobs.

maybe the following is easier to understand and more aligned with 
bitmap-mode:

The name of the bitmap to use for sync=bitmap/sync=incremental mode. 
Must be present if sync is "bitmap" or "incremental". Must NOT be 
present otherwise.

>> +#          granularity is used instead of @granularity (since 5.2).
>> +#
>> +# @bitmap-mode: Specifies the type of data the bitmap should contain after
>> +#               the operation concludes. Must be present if sync is 
>> "bitmap".
>> +#               Must NOT be present otherwise. (Since 5.2)

Specifies the type of data the bitmap should contain after the operation 
concludes. Must be present if sync is "bitmap". Must be "on-success" or 
absent if sync is "incremental". Must NOT be present otherwise.

>> +#
>>  # @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
>> -#               power of 2 between 512 and 64M (since 1.4).
>> +#               power of 2 between 512 and 64M. Must not be specified if
>> +#               @bitmap is present (since 1.4).
>>  #
> 
> Is @granularity forbidden with @bitmap because it makes no sense?

yes.

> 
> If yes, then it doesn't actually default to anything then, does it?

we must use the same granularity as the sync bitmap passed in via 
'bitmap', so the caller can't set a different one.

> Looks like we have
> 
>     sync            'bitmap'            anything else
>     -------------------------------------------------
>     bitmap          required            forbidden
>     bitmap-mode     required            forbidden
>     granularity     forbidden           optional
> 
> Correct?

yes. with the addition of sync=incremental as subset of sync=bitmap, as 
described above.

> 
>>  # @buf-size: maximum amount of data in flight from source to
>>  #            target (since 1.4).
>> @@ -2002,7 +2011,9 @@
>>  { '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',
>> +            '*bitmap-mode': 'BitmapSyncMode',
>> +            '*mode': 'NewImageMode',
>>              '*speed': 'int', '*granularity': 'uint32',
>>              '*buf-size': 'int', '*on-source-error': 'BlockdevOnError',
>>              '*on-target-error': 'BlockdevOnError',
> [Same for blockdev-mirror...]
> 
> 
> 
>