Re: delalloc fragmenting files?

[Eric Sandeen <sandeen@redhat.com>]

Andreas Dilger wrote:
> On Nov 06, 2007  13:54 -0600, Eric Sandeen wrote:
>> Hmm bad news is when I add uninit_groups into the mix, it goes a little
>> south again, with some out-of-order extents.  Not the end of the world,
>> but a little unexpected?

> I think part of the issue is that by default the groups marked BLOCK_UNINIT
> are skipped, to avoid dirtying those groups if they have never been used
> before.  This policy could be changed in the mballoc code pretty easily if
> you think it is a net loss.  Note that the size of the extents is large
> enough (120MB or more) that some small reordering is probably not going
> to affect the performance in any meaningful way.

You're probably right; on the other hand, this is about the simplest
test an allocator could wish for - a single-threaded large linear write
in big IO chunks.

In this case it's probably not a big deal; I do wonder how it might
affect the bigger picture though, with more writing threads, aged
filesystems, and the like.  Just thought it was worth pointing out, as I
started looking at allocator behavior in the simple/isolated/unrealistic
:) cases.

-Eric