**This article is for Isilon administrators or who have the experience of Isilon product.**

Isilon is a scale-out NAS storage doing an excellent work of scale out the ingesting data across the nodes within the single namespace(a very very large Peta bytes partition).

For the protection mechanism, Isilon used the RAIN(redundancy node) instead of the RAID(redundancy disk) to enhance the data protection to the next level protection.

To name a few before we discuss the protection, Isilon has the following definition for redundancy, called N+M mechanism.

First of all, Isilon is a cluster so we must at least to have three nodes setup initially.

Secondly, is the definition of N and M (those are the restrictive conditions must be followed):

- A data stripe is a 128KB strip unit to store data or FEC.
- N must be greater than M (N>M)
- N is the first digit to specify the number of data stripe units in a stripe. The maximum N is 16.
- M is the second digit to specify the number of FEC (forward error correction) stripe units in a stripe. The maximum M is 4.

So let’s review this table gathered from any one version of OneFS administration guide. So how to do the following calculation:

I will use the **fifth column** as an example.

So second row 3 nodes cluster situation, since the protection level set with +3d:1n(bear with one node down or 3 simultaneous disks failure at any one of nodes).

In this circumstances, each node store three data stripes to three different disks simultaneous when new writes go in since our protection set with +3d:1n(you ask me why goes three stripe for each node, that’s really a great question, please image by yourself to figure it out, in which case Isilon could survive in any condition when three disks failed, please draft out the data layout by yourself), that means we need to have 1 FEC protection data stripe generated. Since we write 3 disks for each node, we will do the following:

2+1 ===> 2*3+1*3 = 6+3 (ohhh, that’s really the number in that table.)

So the next question:

From row 5 and row 6, why the numbers are the same now (15+3)?

That’s a tricky question, please review the restrictive conditions and you will find out the answer 🙂

The third question is why from row fourteenth the number changed from 15+3 to 16+3?

I will give you some hits here, check column seventh and understand restrictive conditions.

If you could answer those two questions, I believe you could explain the why dashes (-) show on the table as well.

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