Reviewing ext3 improvement patches (delalloc, mballoc, extents)

[Suparna Bhattacharya <suparna@in.ibm.com>]

Since the performance improvements seen so far are quite encouraging, 
and momentum is picking up so well, I started looking through the
patches from Alex ... just a quick code walkthrough to get a hang
of it and think about what kind of simplifications might be possible
and what it might take for inclusion.

I haven't had a chance to go too deep line by line yet,
but thought I'd initiate some discussion with some first impressions
and summary of what directions I hear several people converging
towards to validate if I'm on the right track here.

diffstat of the 3 patches : 22 files changed, 5920 insertions(+), 
47 deletions. The largest is in the extents patch (2743), mballoc 
is 1968, and delalloc is 1209. To use delalloc, which gives us
all the performance benefits, right now we need all the 3 patches
to be used in conjunction. Supporting extent map btrees as well 
as traditional indexing and associated options for compatibility etc
is perhaps the more invasive of changes. Given that keeping ext3 
stable and maintainable is a key concern (that is after all a major 
reason why a lot of users rely on ext3), a somewhat incremental 
approach is desirable. 

So, I'll start from the direction that has been suggested by
some -- (1) delayed allocation without changing the
on-disk format. And then later (2) go on to breaking format with 
all changes for scalability to larger files with full extents 
support (haven't thought enough about this yet - maybe in a
separate mail)

A few random things that come to mind for (1), going through the code:

- There might be possibilities for code reduction, by extending
  generic routines as far as possible, e.g. ext3_wb_writepages
  has a lot in common with generic writepages. That would
  also make it easier to maintain.
- Similarly, how about (as Mingming I think already hinted) 
  implementing ext3_get_blocks to do multi-block lookup and 
  allocation and using it in delalloc ?

Hmm, maybe I speak too soon - have to look at the interfaces more
closely and verify if this is feasible. 

Regards
Suparna


-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India



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