Re: [PATCH v7 1/3] block: introduce compress filter driver

[Vladimir Sementsov-Ogievskiy]

15.11.2019 13:52, Vladimir Sementsov-Ogievskiy wrote:
> 15.11.2019 12:32, Max Reitz wrote:
>> On 14.11.19 12:59, Vladimir Sementsov-Ogievskiy wrote:
>>> 14.11.2019 14:27, Max Reitz wrote:
>>>> On 13.11.19 19:43, Andrey Shinkevich wrote:
>>>>> Allow writing all the data compressed through the filter driver.
>>>>> The written data will be aligned by the cluster size.
>>>>> Based on the QEMU current implementation, that data can be written to
>>>>> unallocated clusters only. May be used for a backup job.
>>>>>
>>>>> Suggested-by: Max Reitz <address@hidden>
>>>>> Signed-off-by: Andrey Shinkevich <address@hidden>
>>>>> ---
>>>>>    block/Makefile.objs     |   1 +
>>>>>    block/filter-compress.c | 201 
>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>    qapi/block-core.json    |  10 ++-
>>>>>    3 files changed, 208 insertions(+), 4 deletions(-)
>>>>>    create mode 100644 block/filter-compress.c
>>>>>
>>>>> diff --git a/block/Makefile.objs b/block/Makefile.objs
>>>>> index e394fe0..330529b 100644
>>>>> --- a/block/Makefile.objs
>>>>> +++ b/block/Makefile.objs
>>>>> @@ -43,6 +43,7 @@ block-obj-y += crypto.o
>>>>>    block-obj-y += aio_task.o
>>>>>    block-obj-y += backup-top.o
>>>>> +block-obj-y += filter-compress.o
>>>>>    common-obj-y += stream.o
>>>>> diff --git a/block/filter-compress.c b/block/filter-compress.c
>>>>> new file mode 100644
>>>>> index 0000000..64b1ee5
>>>>> --- /dev/null
>>>>> +++ b/block/filter-compress.c
>>>>> @@ -0,0 +1,201 @@
>>>>> +/*
>>>>> + * Compress filter block driver
>>>>> + *
>>>>> + * Copyright (c) 2019 Virtuozzo International GmbH
>>>>> + *
>>>>> + * Author:
>>>>> + *   Andrey Shinkevich <address@hidden>
>>>>> + *   (based on block/copy-on-read.c by Max Reitz)
>>>>> + *
>>>>> + * This program is free software; you can redistribute it and/or
>>>>> + * modify it under the terms of the GNU General Public License as
>>>>> + * published by the Free Software Foundation; either version 2 or
>>>>> + * (at your option) any later version of the License.
>>>>> + *
>>>>> + * This program is distributed in the hope that it will be useful,
>>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>>>> + * GNU General Public License for more details.
>>>>> + *
>>>>> + * You should have received a copy of the GNU General Public License
>>>>> + * along with this program; if not, see <http://www.gnu.org/licenses/>.
>>>>> + */
>>>>> +
>>>>> +#include "qemu/osdep.h"
>>>>> +#include "block/block_int.h"
>>>>> +#include "qemu/module.h"
>>>>> +
>>>>> +
>>>>> +static int compress_open(BlockDriverState *bs, QDict *options, int flags,
>>>>> +                         Error **errp)
>>>>> +{
>>>>> +    bs->backing = bdrv_open_child(NULL, options, "file", bs, 
>>>>> &child_file, false,
>>>>> +                                  errp);
>>>>
>>>> Please don’t attach something that the QAPI schema calls “file” as
>>>> bs->backing.
>>>
>>>
>>> Agree, it's a mistake. If we want backing and user set backing in options, 
>>> it's opened automatically, I think..
>>>
>>>>
>>>> Yes, attaching it as bs->file would break backing chains.  That’s a bug
>>>> in the block layer.  I’ve been working on a fix for a long time.
>>>>
>>>> Please don’t introduce more weirdness just because we have a bug in the
>>>> block layer.
>>>>
>>>> (Note that I’d strongly oppose calling the child “backing” in the QAPI
>>>> schema, as this would go against what all other user-creatable filters do.)
>>>>
>>>
>>> So, are you opposite to correct backing-based user-creatable filter (with 
>>> backing both
>>> in QAPI and code)?
>>
>> I’m not opposed to fixing it, but I don’t think the fix is to make all
>> filters use bs->backing.
>>
>>> Do you think, that if we make backup-top to be user-creatable, we should 
>>> move it to be
>>> file-child-based, or support both backing and file child?
>>
>> I definitely don’t think it would be wrong.
>>
>> It depends on how difficult it is.  I’m currently working on (more
>> groundwork for the filter series v7) a series to rework BdrvChildRole so
>> we can see from it what a child is used for (data, metadata, filter,
>> COW).  I can already see that it won‘t work out perfectly because
>> whenever we attach "backing", the question is whether that’s a COW child
>> now or whether it’s a filtered child.  I suppose I’m going to guess COW
>> when there’s no way to get the information, and maybe sometimes be wrong.
>>
>> In my honest opinion, reusing bs->backing for filters was wrong.  I’m
>> not saying that bs->file was any better.  But I have a bit of a gripe
>> with filters using bs->backing, because it’s acknowledging a bug but not
>> fixing it at the same time.  Had we fixed the bug when we first noticed
>> it with the introduction of the mirror filter, maybe we wouldn’t be in
>> this position now.  Or maybe we should have just added a bs->filtered link.
>>
>> But maybes aside, it still means that using bs->backing instead of
>> bs->file is not really better.  Right now it’s both wrong, and we need
>> to fix the block layer so it isn’t.
>>
>> So what to do for new filters?  Sure, bs->backing works around a bug
>> now.  But it’ll be weird once the bug is fixed.  Then we’ll have filters
>> that use @file and others will use @backing.  I don’t think we want
>> that, I think we want a uniform interface for all filters.
>>
>> And yes, that implies we probably should change backup-top to use file
>> instead of backing once it gets an external interface.
>>
>> (Compare
>> https://lists.nongnu.org/archive/html/qemu-block/2017-09/msg00380.html
>> )
>>
>> Max
>>
> 
> OK, got your point. Let's use file child in compress filter. Hope for your 
> series!
> 

Interesting, how much of your series needed to make it possible to use compress 
filter
in stream? To make it work in 5.0?

-- 
Best regards,
Vladimir