linux-ext4 - Re: fine grain block allocation API request

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Message-ID: <8761hvsh13.fsf@openvz.org>
Date:	Thu, 14 Aug 2014 10:48:40 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Theodore Ts'o <tytso@....edu>,
	Lukas Czerner <lczerner@...hat.com>
Subject: Re: fine grain block allocation API request

On Wed, 13 Aug 2014 22:38:06 +0200, Andreas Dilger <adilger@...ger.ca> wrote:
> One question to ask is whether a goal inode allocation API would be enough
> or if you need specific block allocation, or both? If you could start
> allocation at a specific inode, would that be enough to avoid the messy
> code below?
In order to pack small files I need goal block allocator API.
Goal inode allocation API is almost useless for my task. But other tasks may
gain from that feature. For example this allow to implement online filesystem shrink.
 
> 
> Cheers, Andreas
> 
> > On Aug 13, 2014, at 15:45, Dmitry Monakhov <dmonakhov@...nvz.org> wrote:
> > 
> > 
> > Currently we have not public API for block allocation from some
> > specific region (group/flex_group/blk_range).  The only existing
> > hack is available in migrate.c where we use goal_inode for
> > ext4_new_inode().
> > 
> > In fact this tend make some specific task very complicated.
> > For example my e4defrag2 utility try to compact small files from
> > one group together. In order to allocate blocks from given group
> > I have to perform number of crazy stuff, such as:
> > 1) Prepare cache of directories according to it's group (where dir_inode
> > is placed)
> > 
> > 2) Try to allocate local donor file with local blocks (from given group)
> >   Pseudo code of donor creation procedure :
> >   int find_donor(int grp, ...) {
> >       for (i = 0; i = dir_cache[grp].num;i++) {
> >           fd = openat(dir_cache[grp].fd[i], "donor", O_CREAT|flags,... )
> >           fstat(fd, &stat);
> >           if (inode_group(stat->st_ino) != grp)
> >              goto next_candidate;
> >           do_falloc(fd, size);
> >           /* Check that blocks we allocated are belongs to group we want*/
> >           if (!is_block_local(fd, group))
> >              goto next_candidate;
> >           goto found_donor;
> >          .....
> >       }
> >      .....
> >   }
> > All this machinery is very ugly, unstable and increase code-base about
> > 500-700 lines of code. And I want to ask you why we do not have such
> > API? AFAIR this API was requested several times, but it was rejected.
> > I do understand that this is not good idea to allow unprivileged user to
> > manipulate block allocator, but IMHO we can hide it under CAP_RESOURCES
> > or CAP_ADMIN.
> > Please tell be your opinion. I'll happy to implement that ioctl.
> > --
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