[FAQ] XFS speculative preallocation

[Eric Sandeen]

On 3/21/14, 6:10 PM, Dave Chinner wrote:
> On Fri, Mar 21, 2014 at 09:11:29PM +0100, Florian Weimer wrote:
>> * Brian Foster:
>>
>>> Although speculative preallocation can lead to reports of excess space
>>> usage, the preallocated space is not permanent unless explicitly made so
>>> via fallocate or a similar interface.
>>
>> How does an explicit allocation with posix_fallocate interact with
>> speculative preallocation?  Does it disable it?
> 
> fallocate is permanent preallocation using unwritten extents.
> Speculative preallocation is an extension of delayed allocation that
> is done when extending the file and the EOF falls into a hole. If
> there is unwritten extents beyond EOF, speulative preallocation is
> not performed.
> 
>> I see rather dramatic fragmentation of the systemd journal when it is
>> stored on XFS, and it calls posix_fallocate before writing data to the
>> file.
> 
> There's your problem - systemd is preventing delayed allocation, and
> so it fragmenting the file itself with it's write pattern.
> Basically, that's a bug in systemd, and not something the filesystem
> can avoid because userspace is directly controlling block
> allocation.

hohum, I guess we should look into this.

OTOH: nothing wrong with calling posix_fallocate() if you need the space
guarantees it provides for proper operation...

-Eric

> Cheers,
> 
> Dave.
>