# RAID 5 Array: Probability of Failing to Rebuild Array

I have a Qnap ts463u-rp data storage array, which contains 4x4TB HDD.

my question here is, is it safe to configure 3x4TB as RAID 5?

because from what I understood, it's not recommend to configure 12TB storage with RAID 5, because of the Unrecoverable Read Error when rebuilding the Array. is this Right?

It's not necessarily bad to configure your RAID 5 array with N TB. What you need to worry about is how much data has to be read AFTER you lose a drive. Depending on the amount and size of the disks in your RAID 5 array, is what calculates your probability of failure for rebuilding the array.

What is a URE?

A URE (Unrecoverable Read Error) is when reading the sector of a drive fails, and it cannot be fixed. A URE number simply means that "on average 1 error is detected while reading n bits". In our scenario, it's simply running a risk that it will fail to be able to read one of the distributed parities while rebuilding the array.

How to find out my probability of failure?

A typical consumer grade hard drive has a bit error rate of 10¹⁴. To calculate the probability, we would use the formula `Probability = 1-((X-1)/X)^R`. X is the number of outcomes, and R is the number of trials. In this case X will be our bit error rate for the drive, 10¹⁴. R will be the size of the RAID array after losing a disk. For this case that is 8 TB.

Math Time!

First, we want to convert 8 TB to bits. This equals 6.4*10¹³. Now we can finally calculate the probability of our RAID 5 failing to rebuild after losing a disk.

Probability = 1-((10¹⁴-1)/10¹⁴)^(6.4*10¹³) ... plug this bad boy into wolfram alpha and you get 0.4727... multiply that by 100 and you have a 47% chance of your RAID 5 array failing to rebuilding in a 3x4TB setup that loses a drive. If you have a 4x4TB setup and you lose one drive, you have a 61% of the disks failing to rebuild.

So should I risk it?

The overall conclusion is that if you are using consumer grade hard drives, it can be risky to use large drives with RAID 5. It's a much different story with enterprise grade drives and hardware. As an example, using a Seagate Enterprise drive with a bit error rate of 10¹⁵, recovering a 4x4TB RAID 5 array that loses a drive, has only a 9% chance of failing to rebuild. The consumer grade hard drives has a 61% chance of failure.

In response to a comment by Christoper, I found this sleek RAID rebuild failure chance calculator. It quickly and easily calculates the probability of a URE failing during a RAID 5 or RAID 6 rebuild.

RAID rebuild failure chance calculator - created and maintained by magJ

• Neat! Do you have a source and/or similar information for RAID10? Jun 29, 2018 at 15:11
• @ChristopherHostage no I don't. But thanks to your comment I did some looking around and happen to stumble upon a nice calculator for calculating the probability of failure for RAID 5 and RAID 6. Jun 29, 2018 at 15:29
• @ChristopherHostage I showed this post to some co-workers today because this topic came up. I somehow stumbled upon a Reddit post that shows how to calculate the failure rate for Raid10 due to UREs Nov 5, 2018 at 20:54
• Excellent answer, thank you. I am, however, curious - what happens after an URE? Is the whole array lost, all the data "after" the URE or just files residing on blocks affected by the URE? May 8, 2019 at 20:15
• @quantum If a RAID5 system experiences a URE during rebuild, is all the data lost? I believe that will answer your questions. May 8, 2019 at 21:20