My setup is the following:
Synology Disk Station as iSCSI target
OS X 10.8 as client
Debian as client
I don't like to know that I should use WebDAV or similar file sharing protocols.
I'd like to access the same target from at least the both above named devices. What file system should I use? I guess it should be a cluster file system, but which one should I use for best compatibility?
If you're going to simultaniously access the LUN from both debian and OS X you'll need a clustered filesystem like GFS.
This means that you'd have to cluster your OS X with the debian box, and that sounds to me like a really bad idea. I doubt if it's even doable.
Since your Synology Disk Station is a single point of failure already, clustering the file system will not save you from problems, but is likely to introduce them instead.
You'll save yourself a lot of headache by accessing the files through something like NFS.