ck's archive: raid decision

Due to some drama with eddie's RAID array, I've got four 18GB 15,000 RPM SCSI disks that I'm going to put into zem. I'm currently pondering what RAID level would best make use of them. The contenders:

1. RAID 5

   • 36GB usable with three disks in the array and a hot spare.
   • Hot spare disk does much less work than the other three, so should last longer.
   • Can lose any disk and then any other disk (after the hot spare syncs up) and still function.

2. RAID 6

   • 36GB usable with four disks in the array.
   • Read performance is striped across four disks, so it's the absolute fastest of the options.
   • Can lose any two disks and still function.

3. RAID 10

   • 36GB usable with four disks in the array.
   • Best write performance of the three, worst read performance (though still very good).
   • Only RAID level among the three to offer true data redundancy.
   • Can lose any disk, and then either disk from the other mirrored pair and still function.

Since zem is primarily a web server, read performance matters more than write performance. Right now, it looks like RAID 6 and RAID 10 are fighting for the lead, since RAID 6 beats RAID 5 in everything but the expected longevity of the fourth disk.

It's also worth noting that I'll be doing this in software, so RAID 6's parity calculation is a slight handicap on writes.

Which should I pick? What did I overlook?