Re: Proposal and plan for ext2/3 future development work

[Joel Becker <Joel.Becker@oracle.com>]

On Fri, Jun 30, 2006 at 10:13:06AM -0700, Badari Pulavarty wrote:
> I tried adding "delayed allocation" for ext3 earlier. Yes. VFS level
> infrastructure would be nice. But, I haven't found much that we can
> do at VFS - which is common across all the filesystems (except
> mpage_writepage(s) handling). Most of the stuff is specific to 
> filesystem implementation (even though it could be common) - coming
> out with VFS level interfaces to suite all the different filesystem
> delalloc would be *interesting* exercise.

	Well, to be fair, I'm just going by what little I know about
XFS.  They maintain a cache of all pages waiting on delayed allocation
for writepack.  Why have this entire cache (hash, list, whatever) when
we could create some state on in the pagecache?  We save a large chunk
of memory and some complex writeback code.  I suspect you were thinking
of this when you said "mpage_writepage(s) handling".  But this is a
large complexity win if we can do it.
	The same with metadata/data ordering issues.  ie, data=ordered
or even plain "creat(2); write(2)".  I don't know how generic the
ordering is for each filesystem, but there is always room for play.
	On-disk, of course each filesystem is going to be different.
I'm not sure we could fit a fully-generic aops->reserve_space() &
aops->commit_space() API.  But I don't think we need to.

Joel
-- 

"Well-timed silence hath more eloquence than speech."  
         - Martin Fraquhar Tupper

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127