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Re: [Qemu-block] [PATCH 16/22] qmp: add persistent flag to block-dirty-b

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH 16/22] qmp: add persistent flag to block-dirty-bitmap-add
Date: Mon, 24 Oct 2016 19:30:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 24.10.2016 17:12, Vladimir Sementsov-Ogievskiy wrote:
> 10.10.2016 19:08, Max Reitz wrote:
>> On 30.09.2016 12:53, Vladimir Sementsov-Ogievskiy wrote:
> [...]
>>> +    }
>>>      out:
>>>       aio_context_release(aio_context);
>>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>>> index 31f9990..2bf56cd 100644
>>> --- a/qapi/block-core.json
>>> +++ b/qapi/block-core.json
>>> @@ -1235,10 +1235,15 @@
>>>   # @granularity: #optional the bitmap granularity, default is 64k for
>>>   #               block-dirty-bitmap-add
>>>   #
>>> +# @persistent: #optional the bitmap is persistent, i.e. it will be
>>> saved to
>>> +#              corresponding block device on it's close. Default is
>>> false.
>>> +#              For block-dirty-bitmap-add. (Since 2.8)
>> I'm not sure what the "For block-dirty-bitmap-add." is supposed to mean,
>> because this whole struct is for block-dirty-bitmap-add (and for
>> block-dirty-bitmap-add transactions, to be exact, but @persistent will
>> surely work there, too, won't it?).
>> Also, I'd say "will be saved to the corresponding block device image
>> file" instead of just "block device", because in my understanding a
>> block device and its image file are two separate things.
> Hmm.. But 'its close' is block device close, not file close.

In my understanding, it is the file close.

>                                                              And we call
> common bdrv_* function to save it, so we actually save it to device, and
> then the device puzzles out, how to actually save it.

Well, OK, it depends on what you mean by "block device". There are many
things we call a "block device", but nowadays I think it mostly refers
to either a guest block device or a BlockBackend (and as of lately,
we're more and more trying to hide the BlockBackend from the user, so
you could argue that it's only the guest device now).

The bdrv_* functions operate on block layer BDS nodes, and I don't think
we call them "block devices" (at least not any more).

In any case, I think for users the term "block device" refers to either
the device presented to the guest or to all of the block layer stuff
that's underneath, and it's not quite clear how you could save a bitmap
to that, or what it's supposed to mean to "close" a block device (you
can remove it, you can destroy it, you can delete it, but "close" it?).

But saying that it will be saved to the image file that is attached to
the block device will make it absolutely clear what we mean.

So what you have called a "device" here is neither what I'd call a
device (I'd call it a "node" or "BDS"), nor what I think users would
call a device. Also, it's not the BDS that puzzles out how to save it
but some block driver.

>>> +#
>>>   # Since 2.4
>>>   ##
>>>   { 'struct': 'BlockDirtyBitmapAdd',
>>> -  'data': { 'node': 'str', 'name': 'str', '*granularity': 'uint32' } }
>>> +  'data': { 'node': 'str', 'name': 'str', '*granularity': 'uint32',
>>> +  '*persistent': 'bool' } }
>> I think normally we'd align the new line so that the opening ' of
>> '*persistent' is under the opening ' of 'node'.
>>>     ##
>>>   # @block-dirty-bitmap-add
>>> diff --git a/qmp-commands.hx b/qmp-commands.hx
>>> index ba2a916..434b418 100644
>>> --- a/qmp-commands.hx
>>> +++ b/qmp-commands.hx
>>> @@ -1441,7 +1441,7 @@ EQMP
>>>         {
>>>           .name       = "block-dirty-bitmap-add",
>>> -        .args_type  = "node:B,name:s,granularity:i?",
>>> +        .args_type  = "node:B,name:s,granularity:i?,persistent:b?",
>>>           .mhandler.cmd_new = qmp_marshal_block_dirty_bitmap_add,
>>>       },
>>>   @@ -1458,6 +1458,9 @@ Arguments:
>>>   - "node": device/node on which to create dirty bitmap (json-string)
>>>   - "name": name of the new dirty bitmap (json-string)
>>>   - "granularity": granularity to track writes with (int, optional)
>>> +- "persistent": bitmap will be saved to corresponding block device
>>> +                on it's close. Block driver should maintain
>>> persistent bitmaps
>>> +                (json-bool, optional, default false) (Since 2.8)
>> And I don't know what the user is supposed to make of the information
>> that block drivers will take care of maintaining persistent bitmaps. All
>> they care about is that it will be stored in the corresponding image
>> file, so in my opinion it would be better to just omit the last sentence
>> here.
> Users shoud know, that only qcow2 supports persistent bitmaps. Instead
> of last sentence I can write "For now only Qcow2 disks supports
> persistent bitmaps".

s/supports/support/, but yes, that sounds preferable to me.


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