linux-ext4 - Re: [RFC] Heads up on sys

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Message-ID: <45EC8E6C.6040403@redhat.com>
Date:	Mon, 05 Mar 2007 15:41:00 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Mingming Cao <cmm@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, nscott@...nex.com,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, suparna@...ibm.com, alex@...sterfs.com,
	suzuki@...ibm.com, Ulrich Drepper <drepper@...hat.com>
Subject: Re: [RFC] Heads up on sys_fallocate()

Jan Kara wrote:

>> I am wondering if it is useful to add another mode to advise block 
>> allocation policy? Something like indicating which physical block/block 
>> group to allocate from (goal), and whether ask for strict contigous 
>> blocks. This will help preallocation or reservation to choose the right 
>> blocks for the file.
>   Yes, I also think this would be useful so you can "guide"
> preallocation for things like defragmentation (e.g. preallocate space
> for the file being defragmented and move the file to it).

Hints & policies for allocation would certainly be useful, but I think
they belong outside this interface.  i.e. you could flag an inode for
whatever allocation you choose, and -then- call posix_fallocate so that
the allocator will take the hints you've given it.

See also this blurb from the posix_fallocate definition:

"It is implementation-defined whether a previous posix_fadvise() call
influences allocation strategy."

FWIW I don't see a lot of point in asking for "strict contiguous blocks"
- the allocator will presumeably try to do this in any case, and I'm not
sure when you would want to fail if you get more than one extent...?

-Eric
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