How does RAID actually work?

[Arokan]

Hey LTT-Community,

 

I've been reading all I found about RAID and I'm thinking about using it in the future.

However, there is something I cannot wrap my head around.

 

When using RAID-5, it's stated that you can lose one out of tree drives before you lose any data, with two drives as storage and one drive to rebuild the array.

 

Does it matter, which drive you lose and if so, wouldn't that be as risky? And if it doesn't matter which drive you lose, how is the storage arranged for that to be possible?

Looking forward to your answers,

Arokan

[Trinopoty]

In overly simple terms, imagine you have two numbers: 25 and 67. Now you can add them to get 92. Raid 5 stores these 3 numbers in 3 drives. So even if you lose, say, 25; you can get it back by doing 92 - 67. And, if you replace the drive, the controller goes to every number and restores the missing one.

 

Now, actual RAID5 implementation is quite a lot complicated and it allows you to use pretty much all the storage of the 3 drives.

[DailyProcrastinator]

 

Watch these, I would think this answers all your questions.

[Arokan]

@DailyProcrastinator Thanks, watched those already but it didn't answer all my questions.

 

@Trinopoty Okay, then I start to understand how you can lose one drive and it doesn't matter which one. But how can you set it up so you can use almost all the storage?
I'm happy to learn all about it, complicated as it may be, and I'd be thankful for a link or so to a full explanation as I didn't manage to find one myself. Most are pretty superficial.

[AbydosOne]

19 minutes ago, Arokan said:

Does it matter, which drive you lose

No.

 

19 minutes ago, Arokan said:

how is the storage arranged for that to be possible?

Simplified example:

 

Three disks (D1, D2, D3); D1 and D2 are data disks; D3 contains parity information generated from D1 and D2.

When you write data to the array, half will be put on D1 and the other half will be put on D2 (like RAID 0).

The parity written to D3 will be the bitwise XOR of the data written to D1 and D2.

(XOR = eXclusive OR, 0 XOR 0 = 0, 0 XOR 1 = 1 XOR 0 = 1, 1 XOR 1 = 0)

 

If D3 fails and is replaced, the system generates XOR parity information from existing data from D1 and D2 (easy).

If D1 or D2 fails, you can infer the value on the failed disk from the remaining disk and the D3 (parity) information.

For example, if a bit on D1 is a 0, and the corresponding bit on D3 (parity) is 1, then you know that corresponding bit on D2 was a 1 (per the XOR operation); basically solving D1 XOR n = D3.
A little more computationally complex, but not terrible.

 

EDIT: IIRC, XOR is fully reversible, so D1 XOR D2 = D3 means that D1 XOR D3 = D2, etc.

 

More EDIT: this process is extensible to more drives (assuming you only lose one at a time); XOR can be chained together: D1 XOR D2 XOR D3 = D4, etc.

 

10 minutes ago, Trinopoty said:

In overly simple terms, imagine you have two numbers: 25 and 67. Now you can add them to get 92. Raid 5 stores these 3 numbers in 3 drives. So even if you lose, say, 25; you can get it back by doing 92 - 67. And, if you replace the drive, the controller goes to every number and restores the missing one.

See my explanation above. You've got somewhat the right idea, but you have to remember storage is bit-binary and not decimal (so the "math" is a little different).

[Arokan]

@AbydosOneOkay, got it, thank you very much!

 

[WkdPaul]

* thread moved to the Storage Devices section *

[Archer42]

3 hours ago, Arokan said:

I've been reading all I found about RAID and I'm thinking about using it in the future.

I am going to throw in typical "raid is not backup" warning.

 

If you are planning to use raid at home - stop and think about what you are trying to accomplish and what raid actually does.

 

In short raid is needed to increase availability, trying to eliminate potential downtime due to disk failures.

 

What it does not do -  it does not guarantee data safety. Arrays still can fail for various reasons and data still can be lost in multiple ways. In a sense it can be even more risky than single drive because of increased complexity.

 

So in most cases raid at home is not needed, backup is. And often it is just a matter of choice, same hardware (extra hdd-s) can be used to do either one or another, so choose wisely...