Re: [PATCH] concurrent block allocation for ext2 against 2.5.64

[Andrew Morton <akpm@digeo.com>]

Andreas Dilger <adilger@clusterfs.com> wrote:
>
> >    I think it's probably better to just lump all the root-reserved blocks
> >    into as few blockgroups as possible.
> 
> I might disagree here.  One of the reasons for having the reserved blocks
> is to prevent fragmentation, and not necessarily to reserve space for root.
> For the lots of small files cases it makes more sense to leave free space
> in each group to prevent fragmentation at the group level.

Alex's approach effectively makes every blockgroup a little bit smaller.  I
don't expect it will improve fragmentation effects.  Not sure...

> ...
> We could also say that for the purpose of allocating new files in a directory,
> anything more than 95% full is "full" and the inode should be allocated in
> a different group regardless of where the parent is.  It may be that the
> Orlov allocator already has such a heuristic, but I think that is a different
> discussion.

Yes, both find_group_other() and find_group_orlov() do things like that.

But only in 2.5, or in 2.4 with Ted's backport patches.  find_group_other()
in 2.4 forgets to look at the free block count, which is rather sad.