[Dovecot] minimize mbox mdbox fragmentation

[Timo Sirainen]

On Tue, 2010-10-19 at 21:55 -0500, Stan Hoeppner wrote:

> Any chance the mbox/mdbox writer code could be modified to do physical
> preallocation on files to help avoid file(system) fragmentation?

I've been thinking about that before.

> "What you want is _physical_ preallocation, not speculative
> preallocation. i.e. look up XFS_IOC_RESVSP or FIEMAP so your
> application does _permanent_ preallocate past EOF. 

Oh, interesting. I didn't know that was possible. And even better: Linux
has fallocate() that can do it for other filesystems than just XFS. Or
looks like it's only XFS and ext4 (ext3 doesn't support it). I don't
know if other OSes support this. Maybe in future I could make mdbox
support writing to files whose size has been preallocated by actually
writing NUL bytes, but that requires some extra code.

http://hg.dovecot.org/dovecot-2.0/rev/22c81f884032
http://hg.dovecot.org/dovecot-2.0/rev/b884441a713f