Extents in spare files on ext4 not being merged

[Jari Sundell]

http://libtorrent.rakshasa.no/ticket/460#comment:49

Some users of rtorrent reported very high number of extents for files
despite the use of posix_fallocate/fallocate on ext4. (3000+ for 250MB
file, ) A test program showed that the file was contiguous on disk
despite the high number of extents reported by filefrag. (Not sure
what kernel versions they used)

---
Opening file '/mnt/d1/part1.wmv'.
Block size: 4096
File size:  196571350
Block/page count: 47992
   1: log        0   bb78 phys     8800   bb77

Size of the file is 187.5 MB and filefrag reports 1505 extents.
---

So for some reason the extents aren't getting merged when mmaped
regions get sync'ed in a random order to disk.

Jari Sundell
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