I don't recommend using RAID-5 for any sort of write-intensive work at all. RAID-5 performance is notoriously slow for writes, especially small writes, because of the CPU overhead that each write requires. The overhead is worse if the RAID-5 is software-based. Hardware-based RAID-5 is better, but still not as fast as RAID-1. Of course, RAID-1 requires a great deal of extra disk space (double the usable space you need), so you pay for each approach in different ways.
Can you put the OS on RAID-5? Yes, you can, but I believe you'll find that the system's performance will suffer.
Can you divide a RAIDed slice into multiple partitions? You may be able to, depending on the hardware and software you use, but I think you'd be better served by having two or more separate RAID slices, and assigning each to a separate file system.
Does RAID-5 take a long time to rebuild? Short answer: YES. Long answer: It depends on how large the RAID-5 slice is, how many disks it is striped across, the I/O speed of the disks involved, the CPU speed of the system(s) involved, how much other work is underway and lots of other factors. I have seen large, poorly configured RAID-5 slices take days to rebuild.
The optimal configuration for RAID-5 is to have data spread across four to six disks. Fewer than that is not worth it: You may as well consider RAID-1. More than that invites trouble by increasing the likelihood of multiple disk failures. And, more disks require more time to rebuild.
This was first published in June 2005