Re: [RFC] Heads up on sys_fallocate()

[Eric Sandeen]

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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