I need to compare the reliability of different RAID systems with either consumer or enterprise drives. The formula to have the probability of success of a rebuild, ignoring mechanical problems, is simple:

error_probability = 1 - (1-per_bit_error_rate)^bit_read

and with 3 TB drives I get

38% probability to experience an URE (unrecoverable read error) for a 2+1 disks RAID5 (4.7% for enterprise drives)

21% for a RAID1 (2.4% for enterprise drives)

51% probability of error during recovery for the 3+1 RAID5 often used by users of SOHO products like Synologys. Most people don't know about this.

Calculating the error for single disk tolerance is easy, my question concerns systems tolerant to multiple disks failures (RAID6/Z2, RAIDZ3 and RAID1 with multiple disks).

If only the first disk is used for rebuild and the second one is read again from the beginning in case or an URE, then the error probability is the one calculated above squared (14.5% for consumer RAID5 2+1, 4.5% for consumer RAID1 1+2). However, I suppose (at least in ZFS that has full checksums!) that the second parity/available disk is read only where needed, meaning that only few sectors are needed: how many UREs can possibly happen in the first disk? not many, otherwise the error probability for single-disk tolerance systems would skyrocket even more than I calculated.

If I'm correct, a second parity disk would practically lower the risk to extremely low values.

Am I correct?

**Update**

The calculations are (URE probabilities are per bit read!):

1-(1-1e-14)^(2*3e12*8)=38% for RAID5 2+1 because I have to read two 3TB disks to rebuild the third one

1-(1-1e-14)^(3e12*8)=21% for RAID1 because of the RAID10 with 2x3TB + 2x3TB disks, I have to read only the mirror of he failed disk

1-(1-1e-14)^(3x3e12*8)=51% for RAID5 3+1 because I have to read 3 disks to rebuild the 4th.

Another detail: these probabilities refer *at least* one URE, non necessarily only one. They must be read on the other way round: 62%/79%/49% to complete the rebuild without read errors.