Drive Failure Probability Calculator for RAID Arrays

Two numbers decide how nervous you should be about an array, and they are not the same number. The first is the chance that any drive fails, which is close to certain over a long enough horizon and is exactly what parity exists to absorb. The second is the chance that more drives fail than your parity can cover, which is the one that actually costs you the array. This calculator prints both.

How the probability is calculated

Everything here comes from one input, the annualised failure rate, which is the fraction of drives in a population that fail in a year of service. If a drive survives a year with probability 1 - afr, then it survives y years with probability (1 - afr)^y, so the chance one specific drive fails in that window is q = 1 - (1 - afr)^y.

From there the two headline figures follow:

• At least one of n drives fails: 1 - (1 - afr)^(n x y). Every additional drive is another chance to lose one, which is why an eight-bay array trips this figure so much faster than a two-bay.
• The array is lost: a binomial sum. The chance that exactly k of n drives fail is C(n,k) x q^k x (1-q)^(n-k), and the array survives only while k is no greater than the parity count p. Adding up every case where k is greater than p gives the loss figure.

With no parity at all, losing any single drive loses the array, so the two figures collapse into one. That is worth seeing once: set the parity selector to none and watch the loss number jump to match the failure number exactly.

What annualised failure rate should you use?

Published statistics from large operators, covering tens of thousands of drives, put modern 3.5 inch drives between roughly 1.0 and 1.6 percent per year across a fleet. The default here is 1.4 percent because it sits in that band without being optimistic. Individual models scatter widely around it, and a handful of historically bad models have run several times higher.

Two curves distort a flat annual figure. The first is infant mortality: a meaningful share of drive failures happen in the first few months, from handling damage in transit or a manufacturing defect. That is the entire argument for burn-in testing a new drive before trusting it with data, because a drive that survives its first full surface pass and a couple of weeks of service is far more likely to survive the next several years. The second is wear-out, which starts climbing past roughly five years and never comes back down. Our drive reliability chart by model lists the published figures if you want to pick a rate from data rather than a default.

Why the array loss figure is not a forecast

Read this section before you act on the second output. The binomial model above assumes every drive fails independently, and drives in one array are emphatically not independent. They share almost everything that causes failures:

• The same manufacturing batch. Drives bought together often carry sequential serial numbers, so a batch-level defect reaches several members at once.
• The same age. Every drive crosses the wear-out threshold in the same few months, so the risk does not just rise, it rises for all of them together.
• The same heat and vibration. One failing fan, one warm cupboard in summer or one enclosure with poor damping affects the entire array.
• The same workload. They have written and read the same data for the same number of hours.
• The rebuild itself. Reconstructing a failed member means reading every sector of every survivor, for many hours, at the exact moment redundancy is gone. That is the single most stressful thing the array ever does, and it lands on drives that are all the same age.

The independent model understates correlated risk and overstates uncorrelated risk at the same time. It runs high because it assumes a failed drive is never replaced during the window, when in practice you replace and resilver within days. It runs low because the real second failure is far more likely to arrive during that resilver than chance alone predicts. Use these numbers to compare layouts and drive counts against each other, never as a prediction about your array.

The right response to both distortions is the same. Keep spare capacity and, ideally, a cold spare drive on the shelf so the rebuild starts the day the failure does. Buy drives across two orders or two retailers so they do not share a batch. And past roughly 8 to 10 TB per drive, use double parity, because the rebuild window is now measured in days.

How does drive size change the parity you need?

The probability of a drive failing barely moves with capacity. The consequence of one failing moves enormously, because the array is degraded for as long as the rebuild takes and rebuild time scales directly with drive size. A resilver writes the whole failed member back, at a realistic sustained 80 to 150 MB per second once the array is also serving files, and that arithmetic is what turns a capacity decision into a parity decision.

Set the calculator above to a one year window and read the loss figure, then remember that the exposure window during a 20 TB rebuild is two to three days, not a year. The annual probability is not the right scale for that moment. What is right is the observation that during those days you have no redundancy at all under single parity, on drives that are the same age as the one that just died, while reading every sector of all of them. That is why the recommendation flips at roughly 8 to 10 TB per drive, and why a 20 TB Red Pro should never sit in a single parity array regardless of what the annual arithmetic says.

Shingled drives break this table completely. SMR drives overlap tracks, so a random write forces a read-modify-write of a whole zone, and a rebuild that a CMR drive finishes in 30 hours can take an SMR drive several days. Some arrays have dropped the drive for timing out before it finished. Every drive named on this page is CMR, as are the Toshiba N300 and IronWolf lines, but the plain WD Red line historically included SMR models. Check before you buy.

What does a failing drive actually look like?

Drives rarely stop dead. Most give warning through SMART, the self-monitoring system built into every modern drive, and the array that catches the warning replaces a drive on a Tuesday afternoon instead of rebuilding at 3 am with no redundancy.

Four attributes carry almost all of the signal:

• Reallocated sector count. Sectors the drive has already retired and remapped to spares. Any non-zero value deserves attention, and a value that climbs over weeks is a drive on the way out.
• Current pending sector count. Sectors the drive could not read and has not yet been able to remap. This is the one that turns into an unrecoverable read error during a rebuild, which is exactly the wrong moment.
• Offline uncorrectable. Sectors that failed a scan and could not be fixed. Treat as pending sectors that have run out of chances.
• Command timeout and UDMA CRC error count. Frequently the cable or the backplane rather than the drive. Reseat before condemning, because a bad SATA cable has ended more healthy drives than it should have.

Two habits convert those readings into protection. Run a scheduled scrub or patrol read, monthly is a common cadence, so that every sector is read while redundancy still exists and a bad sector is discovered by the scrub rather than by a rebuild. And keep a cold spare on the shelf. A spare WD Red Plus 8TB sitting in a drawer turns the gap between failure and rebuild from a shipping delay into ten minutes, and that gap is the most dangerous period in the life of the array. Give it a full surface pass before shelving it, as covered in our guide to burn-in testing a new drive, so you are not discovering an infant mortality failure at the worst possible moment.

How much does parity actually buy you?

A lot, and more as the array grows. Take six drives at 1.4 percent over five years. The chance that two or more fail in that window is about 5.8 percent under single parity. The chance that three or more fail, which is what it takes to defeat double parity, is roughly 0.6 percent. That is an order of magnitude for the cost of one drive of capacity.

The comparison understates the real gap, because it only counts drives failing. It does not count the unrecoverable read errors that a single-parity rebuild cannot tolerate and a double-parity rebuild can. The RAID capacity calculator prints the read volume for a rebuild alongside the usable capacity for every layout, and our comparison of RAID 5 against RAID 6 works through the trade in full.

There is a cheaper lever than parity, and it is drive count. Six drives generate 50 percent more failure opportunities than four. Four 20 TB drives and eight 10 TB drives hold the same raw capacity, but the eight-drive array doubles the number of things that can break, doubles the power draw, and needs a larger enclosure. Where a build can be done with fewer, larger CMR drives, it usually should be. Our guide to choosing NAS drives covers which lines are CMR and why an SMR drive should never go into an array.

None of this is a substitute for backups

Every number on this page is about one failure mode: a drive dying. Parity handles that one well. It handles nothing else. Deletion, ransomware, a controller writing garbage to every member, a bad firmware update, theft, fire and flood all reach every drive in the enclosure at once, and no parity level on the list above protects against any of them.

Treat the array as the fast working copy and size a real second and third copy against it with the 3-2-1 backup sizing calculator. An array with a 0.6 percent chance of a parity-defeating failure and no backup is still one accidental delete away from total loss, and that probability is not on this page because it depends on you, not on the drives.