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 <mre...@redhat.com> >>>> Signed-off-by: Andrey Shinkevich <andrey.shinkev...@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.shinkev...@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
