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.
http://tinyurl.com/p2wckx5 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. If I have 6 disks in service then each disk has a .93% probability of surviving which gives a 0.93^6 == 65% probability of having no failures in a year. With filesystems such as ZFS some conditions which would be regarded as "failures" under lesser filesystems become correctable errors. So I don't think I've got a great need to be worried here. With RAID-Z2 if a disk dies then the system can cope with a second disk dying or giving read errors on reconstruction. This is even better than RAID-6 (which only copes with hard read errors and disk failures not corruption). -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/