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

[Steve Lord <lord@xfs.org>]

Joel Becker wrote:
> 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.


No, XFS does not do this, when it gets asked to write out a page which is
delayed alloc, it goes and converts the delayed alloc extent to real disk
space. It then uses the page cache/buffer heads to find all the contiguous
pages which it can turn into a singe disk I/O. The code is made more complex
by other possible states for the data. The only information internal to XFS
though is its extent structures.

Steve