Pathological allocation pattern with direct IO

[Peter Grandi]

> [ ... ] application that write large (tens of GB) files using
> direct IO done in 16 MB chunks. They keep the fs around 80%
> full deleting oldest files when they need to store new ones.
> Usually the file can be stored in under 10 extents but from
> time to time a pathological case is triggered and the file has
> few thousands extents (which naturally has impact on
> performance). [ ... ]

They just want *guaranteed* (not just nearly-always) contiguity,
without preallocating, with incremental writes with direct IO,
while keeping the filesystem full at 80%.

And a pony! :-)