In this MTTDL model, the configuration includes N total disks. If the data protection scheme is raidz3, then the minimum N = 1 data disk + 3 parity disks = 4. You can add more data disks to increase the overall available space, so if N=6 then you have 3 data disks + 3 parity disks.

The model uses the Mean Time between Failure (MTBF) as specified in a vendor's datasheet. It also uses a Mean Time to Repair (MTTR) which includes both the logistical repair time and any data reconstruction required. The simple model calculates MTTDL as:

For non-protected schemes (dynamic striping, RAID-0)

MTTDL[1] = MTBF / N

For single parity schemes (2-way mirror, raidz, RAID-1,RAID-5):

MTTDL[1] = MTBF^{2}/ (N * (N-1) * MTTR)

For double parity schemes (3-way mirror, raidz2, RAID-6):

MTTDL[1] = MTBF^{3}/ (N * (N-1) * (N-2) * MTTR^{2})

For triple parity schemes (4-way mirror, raidz3):

MTTDL[1] = MTBF^{4}/ (N * (N-1) * (N-2) * (N-3) * MTTR^{3})

A graph the results for combinations of 12 disks looks like:

The results are consistent with previous MTTDL analysis. The 12-disk Stripe has an MTTDL of 6.7 years, which isn't very good (annualized rate = 15%) whereas the 12 disk 4-way stripe MTTDL is 2.75e+13 years (annualized rate = 3.63e-12%) and the 12 disk raidz3 MTTDL is 1.67e+11 years (annualized rate = 5.99e-10%).

The theory behind raidz3 will allow more parity disks. But at some point, the system design will be dominated by common failures and not the failure of independent disks. I hope this model will be useful for you to evaluate the data retention of your storage system.

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