Theoretical reliability RAID 0, 5, 6, 10.

RAID5/RAID6 - shows how much safer RAID6 RAID5.

RAID5/RAID10 - shows how much safer RAID10 RAID5.

p - the probability of failure HDD; (HDD failure probability in% divided by 100%)

q = 1 - p reliable HDD.

Red shows the probability of failure of RAID as a percentage.

Other colors are shown the reliability of other types of RAID to RAID5.

```
M3 - the first three months of the disk, the probability of failure, 3%;
Y/2 - half a year, the probability of failure, HDD 2%;
1Y - the first year of HDD, the probability of failure, HDD 1,7%;
2Y - second year of the HDD, the probability of failure HDD 8%;
3Y - the third year of the HDD, the probability of failure, HDD 8,7%;
4Y - the fourth year of the HDD, the probability of failure HDD 6%;
5Y - the fifth year of the HDD, HDD failure probability of 7%;
```

Information on the reliability disk of the source is taken: PDF! Failure Trends in a Large Disk Drive Population

M3: 0,09%; Y/2: 0,04%; 1Y: 0,0289%; 2Y: 0,64%; 3Y: 0,7569%; 4Y: 0,36%; 5Y: 0,49%

RAID1 easiest and most reliable array. Backing up can be seen as a sort of RAID1, if you do it once. The slight difference due to the different reliability.

If you want more reliability, you can create multiple backup copies on different devices. Possible percentage of data loss is reduced to:

RAID0 is the easiest and unreliable array. Needed where the data is not that important at the time of such fleeting experiments. This array is used when speed is needed, and the data can be completely lost.

RAID5 long ago, when the HDD were small and expensive, and the trees were large and reliable due to its smaller size and the speed of recovery were acceptable at the time worked for this type of RAID. If you suddenly came a great idea use just such an array, look at the table.

When there will be a problem with one of the HDD, then RAID5 you will be able to lose the whole array. For example, if it is built from the HDD size
1Tb in the recovery rate will be up to 1-2 days, and the failure rate is about 10-12%, when the array is degraded and will not work. In this case it is better to restore the data to an external storage array.

In general, it was inexpensive storage websites.

To date would not advise use RAID5.

RAID6 - ode reliability. RAID5 to RAID6 slower 20-15-10%, depending on the controller. Recall that RAID5 is not very fast. As the number of drive reliability RAID6 catastrophic falls. 8-10 HDD in array to a reasonable limit.

N - are positive even numbers.

RAID10 hard to find a good ground. Primarily due to the ease of recovery. Even if your controller is burned. Among other things it RAID10 reliability increases with the number of disks. For example, I saw running RAID10 c 12 disks, which of them do not work 4. Strangely enough, this situation is not unusual, since likely to remain in working order at the controller 48.5%.
For two or three discs the situation is much better.

probability of failure of RAID 10 is designed in two ways, the first only with the failure of two HDD, the second - the total probability of failure. In total probability includes all possible number of failed HDD in which RAID will continue.

Failure of the two HDD in RAID 10:

The total probability of failure of RAID 10:

The bottom three tables represent a following items:

The first table contains the number of combinations in which RAID will continue.
Left column - number of HDD in RAID. Top title - number of failed HDD.

Example: from 12 HDD in RAID10 out of order 3 HDD. The number of combinations in which the array will continue to work at failure of three HDD - 160.

The second table shows the number of unique combinations for a certain number of HDD in RAID for a set number HDD.
Left column - number of HDD in RAID. Top title - number of HDD which counts the number of unique combinations.

Example: The number of unique combinations for 3 HDD in the array of 12 HDD - 220.

The third table shows the percentages of combinations in which RAID will continue to work and the total number of combinations.
Left column - number of HDD in RAID. The top title - the probability of failure of RAID if its reliability depends only on the combination of HDD and such a test would be carried out.

Example: from 12 HDD in RAID10 out of order 3 HDD.

The probability of a good combination in which RAID array will continue to 72.7%.

These figures should be taken into account, but also keep in mind that we are dealing with the product of the probabilities. So the resulting probability will be less if this will come down three HDD.

The main purpose of these tables is to give an understanding of why there is a growth reliability RAID10 with the number of disks relatively strong fall reliability RAID6 and RAID5.

How can I check the table with a successful combination? Recall that the total number of successful combinations at failure of HDD is (3^m) -1, where m=N/2. Sum up the line with RAID 10 is like us, even if it is an array of 24 disk: 24+264+ ... + 4096 and compare with (3^12) -1.

Discussion about RAID risk

Joerg H. • Hot spares only help you if the array aggressively fails
drives before they completely die. Then the array can copy the
"failed" disk to the hot spare rather than rebuild the data from the
parity. The problem with this is that it's a bit hit and miss, unless
the array is VERY aggressive and even then there isn't a 100%
guarantee that every failure will be detected before it's a hard
failure. So, at the very least you have to plan on a rebuild from
parity no matter what.

In regards to RAID-6 vs other RAID types and disk size. It really is
as simple as that. When you consider the MTTDL (Mean Time to Data
Loss) in a system with 20 x 1TB drives, a 50MB/sec rebuild rate, and
about 15TB of actual data stored on the system you get the following
RAID failure rates:

Year 1: RAID-5 - 13.76% RAID-10 - 0.078% RAID-6 - 0.516%

Year 2: RAID-5 - 25.6% RAID-10 - 0.156% RAID-6 - 1.03%

Year 3: RAID-5 - 36.86% RAID-10 - 0.23% RAID-6 - 1.54

Year 5: RAID-5 - 53.30% RAID-10 - 0.38% RAID-6 - 2.56%

So, as you can see, I stand by my previous suggestion that the rule of
thumb is actually pretty good. With any drive size 1TB or greater, you
want to use something other than RAID-5, period. You can't rely on hot
spares to save you, so you're only options are RAID-10 or RAID-6 in
your typical array. Pick your poison, or spin the chamber, it's up to
you.

Oh, one more point, for a Netapp system RAID-DP numbers are almost
identical to the RAID-10 numbers in terms of MTTDL. Obviously if your
rebuild times are better than 50MB/sec, then your odds get better, and
if it's worse, like on some SATA drives, then your odds get worse, but
50MB/sec are a pretty good average number to work with.

Finally, here are some who would argue that MTTDL isn't a good measure
of reliability. Unfortunately it's the only one we have that in common
usage. For more info on that topic, take a look at the paper "Mean
time to meaningless: MTTDL, Markov models, and storage system
reliability" by Greenan, Plank, and Wylie.