[FAQ] XFS speculative preallocation

[Florian Weimer]

* 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?

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.