Device mapper (dm) is the native Linux tool to create enterprise grade multipath and fail-over devices with that utilize SAN storage and other modern storage features from within Linux. The Device-mapper Resource Page is a good starting point.
As we're using Dell hardware (poweredge 2950), I found the white paper on using dm for multipath I/O rather helpful. It lists some basic knowledge and concepts, for those not familiar with SAN, HBA cards and other devices.
Interestingly, it also mentions that device mapper only became native to Linux with kernel 2.6.13 and backports have been ported to RHEL 4 update 2 as well as SLES 9 SP2. That makes me wonder whether device mapper is included standard in RHEL 4.4, or OEL 4U5 in our case...? Does a backport mean that you can safely add it to your existing installation if it older than update 2? And is it included by standard in any later version or update, or not?