Messages in this thread | | | Date | Fri, 30 Jun 2006 14:17:00 -0500 | From | Steve Lord <> | Subject | Re: Proposal and plan for ext2/3 future development work |
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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.
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