Re: [PATCH v14 10/13] qapi: block-stream: add "bottom" argument

From: Max Reitz <mreitz@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	qemu-block@nongnu.org
Cc: fam@euphon.net, kwolf@redhat.com, qemu-devel@nongnu.org,
	armbru@redhat.com, stefanha@redhat.com,
	andrey.shinkevich@virtuozzo.com, den@openvz.org,
	jsnow@redhat.com
Subject: Re: [PATCH v14 10/13] qapi: block-stream: add "bottom" argument
Date: Fri, 11 Dec 2020 18:24:51 +0100	[thread overview]
Message-ID: <ef0cc817-69c0-017c-bf94-cc19a66d9dc5@redhat.com> (raw)
In-Reply-To: <9e131589-84b5-761b-3e26-c4e8eb32ec09@virtuozzo.com>

On 11.12.20 17:50, Vladimir Sementsov-Ogievskiy wrote:
> 11.12.2020 19:05, Max Reitz wrote:
>> On 04.12.20 23:07, Vladimir Sementsov-Ogievskiy wrote:
>>> The code already don't freeze base node and we try to make it prepared
>>> for the situation when base node is changed during the operation. In
>>> other words, block-stream doesn't own base node.
>>>
>>> Let's introduce a new interface which should replace the current one,
>>> which will in better relations with the code. Specifying bottom node
>>> instead of base, and requiring it to be non-filter gives us the
>>> following benefits:
>>>
>>>   - drop difference between above_base and base_overlay, which will be
>>>     renamed to just bottom, when old interface dropped
>>>
>>>   - clean way to work with parallel streams/commits on the same backing
>>>     chain, which otherwise become a problem when we introduce a filter
>>>     for stream job
>>>
>>>   - cleaner interface. Nobody will surprised the fact that base node may
>>>     disappear during block-stream, when there is no word about "base" in
>>>     the interface.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   qapi/block-core.json           |  8 +++--
>>>   include/block/block_int.h      |  1 +
>>>   block/monitor/block-hmp-cmds.c |  3 +-
>>>   block/stream.c                 | 50 +++++++++++++++++++---------
>>>   blockdev.c                     | 61 ++++++++++++++++++++++++++++------
>>>   5 files changed, 94 insertions(+), 29 deletions(-)
>>>
>>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>>> index 04055ef50c..5d6681a35d 100644
>>> --- a/qapi/block-core.json
>>> +++ b/qapi/block-core.json
>>> @@ -2522,6 +2522,10 @@
>>>   # @base-node: the node name of the backing file.
>>>   #             It cannot be set if @base is also set. (Since 2.8)
>>>   #
>>> +# @bottom: the last node in the chain that should be streamed into
>>> +#          top. It cannot be set any of @base, @base-node or 
>>> @backing-file
>>
>> s/set any/set if any/
>>
>> But what’s the problem with backing-file?  The fact that specifying 
>> backing-file means that stream will look for that filename in the 
>> backing chain when the job is done (so if you use @bottom, we 
>> generally don’t want to rely on the presence of any nodes below it)?
> 
> I just wanted to deprecate 'backing-file' together with base and 
> base-node as a next step. If user wants to set backing file unrelated to 
> current backing-chain, is it correct at all? It's a direct violation of 
> what's going on, and I doubt that other parts of Qemu working with 
> backing-file are prepared for such situation. User can do it by hand 
> later.. Anyway, we'll have three releases deprecation period for people 
> to come and cry that this is a really needed option, so we can support 
> it later on demand.
> 
>>
>> (If so, I would have thought that we actually want the user to specify 
>> backing-file so we don’t have to look down below @bottom to look for a 
>> filename.  Perhaps a @backing-fmt parameter would help.)
> 
> If we decide that 'backing-file' is really needed, than yes we should 
> require backing-fmt to be specified together with backing-file when 
> using new "bottom" interface.
Before I can agree on removing backing-file (or deprecating it), I need 
to know what it’s actually used for.  I actually don’t, though.  The 
only reason I could imagine was because the user wanted to write some 
string into there that is different from base.filename.

(The original commit 13d8cc515df does mention cases like FD passing, 
where qemu has no idea what an appropriate filename would be (it can 
only see /dev/fd/*).  From that, it does appear to me that it’ll be 
needed even with @bottom.)

Max