Re: [Ext2-devel] Reviewing ext3 improvement patches (delalloc, mballoc, extents)

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

On Mon, Mar 14, 2005 at 05:36:58AM -0300, Werner Almesberger wrote:
> Mingming Cao wrote:
> > I agree delayed allocation make much sense with multiblock allocation.
> > But I still think itself worth the effort, even without multiple block
> > allocation.
> 
> On ABISS, we're currently also experimenting with delayed allocation.
> There, the goal is less to improve overall performance, but to move
> the accesses out of the synchronous code path for write(2).
> 
> The code works quite nicely for FAT and ext2, limiting the time it
> takes to make a write call writing new data to about 4-6 ms on a
> fairly sluggish machine (plus about 2-4 ms for moving the playout
> point, which is a separate operation in ABISS), and with eight
> competing best-effort writers who each enjoy write latencies of some
> 8 seconds, worst-case, overwriting old data.
> 
> Of course, this fails horribly on ext3, because it doesn't do anything
> useful with the journal. Another problem is error handling. Since FAT
> and ext2 don't have any form of reservation, a full disk isn't detected
> until it's far too late.
> 
> So, a VFS-level reservation function would indeed be nice to have.
> 
> I looked at ext3 delalloc briefly, and while it did indeed improve
> performance quite nicely, by being tied to ext3 internals, it would
> be difficult to use in the framework of ABISS, where the code paths
> are different (e.g. the prepare/commit functions should be as close
> to no-ops as possible, and leave all the work to the prefetcher
> thread), and which tries to be relatively file system independent.

I'm looking at whether we can do most of it at VFS level ... with
ext3 only taking care of the additional journalling bit - seems
quite feasible. There are two reqs (1) reservation (2) changing
mpage_writepages to use get_blocks(), which don't seem too hard.
ext3 ordered mode will need a bit more thought.

Of course, I haven't looked at how ABISS does delayed alloc -- 
do you have a patch snippet I can look at ?

Regards
Suparna

> 
> - Werner
> 
> -- 
>   _________________________________________________________________________
>  / Werner Almesberger, Buenos Aires, Argentina     werner@almesberger.net /
> /_http://www.almesberger.net/____________________________________________/
> 
> 
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-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India