Is there a Linux filesystem that will allow 2 hosts to mount the same LUN (on a SAN) or a disk at the same time, both with write and read access?
Like Sun QFS.
this can be done with cluster file systems like RedHat's GFS or Oracle's OCFS2 (http://oss.oracle.com/projects/ocfs2/).
However, the modern approach to this and to get more performance out of your two boxes would be to use a parallel file system. With this approach, each of the two machines would have its own LUN, but you would see a single shared file system with striped file contents, comparable to a RAID across the hosts. In this case, I would recommend the Fraunhofer parallel file system FhGFS (http://www.fhgfs.com), because it's very easy to install and performance is typically very good. But as usual, there are also some other solutions available in this category...