On Tue, 25 Jun 2013, James Harper <james.harper@bendigoit.com.au> wrote:
with 5 SAS ports, you can have 4 data disk (yes, a power of 2) and 1 parity for RAID-Z1. Or you can have three data disks (NOT a power of 2) and 2 parity disks for RAID-Z3.
With old hardware like that I would nominate the 5th disk as a hot spare so on failure you have minimum "degraded" time. This assumes that ZFS can handle a hot spare drive.
Page 4 of the above Google Research document shows that years 2 and 3 have annualised failure rates just over 8% while years 4 and 5 have rates of about 6% and 7%. So it seems that there isn't a great increase in risk as the disk gets older.
I don't see anywhere that you answered my question about the generation of your server, but I assumed it's a HP DL385 (and not a HP DL385 G2 or newer), which is 7-8 years old, and I can't see how you could extrapolate google's results out that far. In any case, my concern was more related to the replacement cost of a failed disk. If you plan on using the server for 3 years then with a hot spare you can take a single disk failure with no loss of redundancy or performance. Or maybe ZFS raid can handle a failure with no loss of performance? James