Looking for a simple file systems project to work on? Try one of the following. The contact is the IRC nick of someone on #linuxfs you can talk to to find out more about the project. If you can also be a contact for a project, please add yourself as one.
Update This project has been worked on by multiple people. This is being driven by the POSIX High End Computing Extensions Working Group http://www.opengroup.org/platform/hecewg/ for distributed filesystems.
http://blogs.sun.com/roller/page/bonwick?entry=seek_hole_and_seek_data
http://www.google.com/search?hl=en&lr=&q=+site:lkml.org+seek_hole+seek_data
Contact: cw, valh
Update Eric Lorimer has written an initial implementation for ext3.
The FIEMAP ioctl (http://oss.sgi.com/archives/xfs/2007-04/msg00082.html) is a related project being implemented for several of the main Linux filesystems. This will allow an application to retrieve a full map of the allocation structure of a file, including information about unallocated or unwritten extents, holes, etc. This provides much more information than SEEK_HOLE and SEEK_DATA, though those could be implemented on top of FIEMAP.
http://infohost.nmt.edu/~val/patches/resv_only_patch
Contact: valh
Update I wrote both a lazy atime and a relative atime, see: http://infohost.nmt.edu/~val/patches.html#relatime