Re: Benchmark : ext3 vs reiser4 and effects of fragmentation.

[Alex Zarochentsev <zam@namesys.com>]

On Thu, Sep 16, 2004 at 06:52:48PM +0300, Markus Törnqvist wrote:
> On Thu, Sep 16, 2004 at 07:38:36PM +0400, Alex Zarochentsev wrote:
> 
> >We are not going to add more intelligence to the in-kernel repacker module but
> >design an interface between repacker and user-space programs and implement
> >reisizing alg. in user-space. 
> 
> This sounds a bit fishy; is it about having two repackers then?
> One in kernelspace and one in userspace?
> Will the kernelspace one be obsoleted?

on-line defragmenter requires its part to be in kernel space. at least node
locks cannot be exported _safely_ to the user space, I think.

> 
> >> >unmergable units shouldn't be problem for the reiser4 kernel code.
> >> What are their significance; what do they mean?
> >two extent units (start1, len1), (start2, len2) can be merged into one (start1,
> >len1 + len2) because start1 + len1 == start2.  I think no reiser4 kernel code
> >depends on that mergable extents do not exist.
> 
> Unmergable units are then (start1 + len1) != start2 because the repacker
> messed this up? 

it can be so because of regular fs activity.   Current block allocation (and
flush algorithm) is not good for appending to existing files.  A free space
after well-allocated reiser4 tree or a well-allocated part of the reiser4 tree
(in case of repacker) can be allocated randomly between new data blocks for
those files.

My opinion -- reiser4 is good in (re)allocating slums.  In case of allocation
many small slums it may be not effective.  The idea of the reiser4 repacker is
to create large slums and allow flush code to do its work.

> 
> The kernel finds the extent units from their "old places" without optimizing
> them to one unit?

in some places, yes, those extents will be merged into one. but i agree, we
can't allow number of those units to grow unlimitedly.  IMHO it would be enough
if the repacker will scan units and do what now is done by fsck -- merging of
mergeable units.

> -- 
> mjt

-- 
Alex.