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Subject: Re: [PATCH RFC 5/5] ext4: Add fallocate2() support
To:     Christoph Hellwig <hch@infradead.org>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Wed, 26 Feb 2020 23:05:23 +0300
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On 26.02.2020 18:55, Christoph Hellwig wrote:
> On Wed, Feb 26, 2020 at 04:41:16PM +0300, Kirill Tkhai wrote:
>> This adds a support of physical hint for fallocate2() syscall.
>> In case of @physical argument is set for ext4_fallocate(),
>> we try to allocate blocks only from [@phisical, @physical + len]
>> range, while other blocks are not used.
> 
> Sorry, but this is a complete bullshit interface.  Userspace has
> absolutely no business even thinking of physical placement.  If you
> want to align allocations to physical block granularity boundaries
> that is the file systems job, not the applications job.

Why? There are two contradictory actions that filesystem can't do at the same time:

1)place files on a distance from each other to minimize number of extents
  on possible future growth;
2)place small files in the same big block of block device.

At initial allocation time you never know, which file will stop grow in some future,
i.e. which file is suitable for compaction. This knowledge becomes available some time later.
Say, if a file has not been changed for a month, it is suitable for compaction with
another files like it.

If at allocation time you can determine a file, which won't grow in the future, don't be afraid,
and just share your algorithm here.

In Virtuozzo we tried to compact ext4 with existing kernel interface:

https://github.com/dmonakhov/e2fsprogs/blob/e4defrag2/misc/e4defrag2.c

But it does not work well in many situations, and the main problem is blocks allocation
in desired place is not possible. Block allocator can't behave excellent for everything.

If this interface bad, can you suggest another interface to make block allocator to know
the behavior expected from him in this specific case?

Kirill