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I'm studying for the CCSP exam and part of the course curriculum discusses data resiliency. At one point, the material has a blurb about RAID and it just seems... incorrect.

In most RAID configurations, all data is stored across the various disks in a method known as striping. This allows data to be recovered in a more efficient manner because if one of the drives fails, the missing data can be filled in by the other drives. in some RAID schemes (there are many, known as RAID 0-10, with different levels of performance, redundancy, and integrity, depending on the owner's needs), parity bits are added to the ra data to aid in recovery after a drive failure.

Emphasis added by me.

My understanding is that striping, in and of itself, does NOTHING for data protection. It simply distributes data across multiple drives. So instead of a single drive having to provide all of the required material, some data might exist on "drive A" and some data might exist on "drive B". This benefits speed... but without some sort of mirroring, striping alone will not help.

Is my understanding correct? If not, can someone please elaborate?

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    It's correct, for anything other than raid0, which isn't really a real raid anyway.
    – djsmiley2kStaysInside
    Commented May 8, 2018 at 14:28
  • An answer I made before: superuser.com/questions/1187057/… Even though specific to my controller, some of that information might help you out
    – Eric F
    Commented May 8, 2018 at 14:32

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It's correct, for anything other than RAID0, which isn't really a real RAID anyway.

Imagine you have 3 pieces of data, A, B and C

Now imagine you have 3 disks, with the data stored in a non-raid style (RAID 0):

Disk 1:   Disk 2:   Disk 3:

A         B         C

The loss of any disk would cause the loss of data.

So, you could triplicate the data over all the disks:

Disk 1:   Disk 2:   Disk 3:

ABC       ABC       ABC

Now, you can lose any 2 disks, but you are using 3x the storage!. This is RAID1 (mirroring).

Then we come to the more interesting RAID levels:

Disk 1:   Disk 2:   Disk 3:

(~~~~~ABC+Parity Data~~~~~)

You can lose any single disk, and rescue the data: With RAID5, however, there is also parity data stored stripped across the disks. This, in essence will use 2/3rds of the storage that RAID1 would use.

That said this is a really short explanation and I'm sure someone else will do better.

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  • Your second example is not RAID 1 and is confusing. You cannot achieve RAID 1 of a 3 disk set with only 3 disks. You may want to change your example to show how to go from a single drive to RAID1 with two drives, and then show how RAID5 will work with 3 drives.
    – Todd Curry
    Commented May 8, 2018 at 15:50
  • Sorry, why do you think it's not RAID1?
    – djsmiley2kStaysInside
    Commented May 8, 2018 at 15:51
  • You updated your example. Clearer now.
    – Todd Curry
    Commented May 8, 2018 at 15:55

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