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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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 <mreitz@redhat.com>
>>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>>> ---
>>>>    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 <andrey.shinkevich@virtuozzo.com>
>>>> + *   (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!

-- 
Best regards,
Vladimir