Raid Recovery - What Is Raid?

By Author: Jennifer Connelly
Total Articles: 5
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The acronym RAID stands for: Redundant Array of Inexpensive Disks Today it has become the standard for most servers, SAN, and NAS devices. These storage arrays offer numerous advantages to the user: Large Storage Space - Combining the size of several disks allows the user to store files that are too large to fit on a single hard drive. Performance - Striping data across multiple disks can also increase data throughput. Example: in a RAID 0 which splits all data across two disks to write a 10Mb file only requires 5Mb to be written to each disk. Effectively doubling the speed which data can be read and written to the array.

Redundancy - In RAID 1 and 10 or 1 0 mirrored volumes are used. This allows for any one hard drive to fail and the array will continue to operate on the mirror of that drive. They are effectively clones of each other. And when the failed drive is replaced, it is rebuilt from the good drive/drives.

In RAID 2 through 5 the data is striped, but parity data is used so that any one drive can fail and the array will still function and can be rebuilt. In RAID 6 a double parity is used so that any two ...
... drives can fail and the array will still function without needing recovery. What is a parity?

A simple way to think about it is as a system of counting odds and evens. These blocks of data are made up of ones and zeros. If you put all the data into columns and then count across them you come up with either an odd number of ones or an even number. By simply adding a 1 for an even number of ones or a 0 for odd number you create a block of parity data.

RAID Data Recovery RI If any drive is lost, it can be recovered by using the odds and evens combined with the data on the remaining drives. The actual system used in RAID is somewhat more complex, but this gives you the basic concept involved.

Choosing Which RAID Array type to Choose This depends on what you're trying to accomplish. I've broken it down by array types and the pros/cons of each type. Take each item into consideration before deciding. RAID 0 - This is a basic stripe array with no redundancy. Pros - Fast Speed & Uses full storage capacity of all disks Cons -

Any one drive fails the data is gone, no redundancy RAID 1 - Simple mirror array Pros - Can sustain one drive failure Cons - No speed advantage over single drive & Expensive (must buy double the number of drives)

RAID 2 through 4 - (Obsolete) - If you have a modern controller it doesn't even support these. They are all similar to 5, however used a single fixed parity drive. The problem is that a fixed parity drive is extremely prone to premature failure caused by the extra read/writes during operation.

RAID 5 - Stripe Array with a single floating parity Pros - Can sustain failure of any one drive, speed advantage similar to 0, but only "wastes" the size of a single disk. Cons - Can't sustain more than one simultaneous failure, not quite as fast as RAID 0 RAID 6 - Stripe array with a double floating parity Pros - Can sustain two simultaneous failures of any two drives, some speed advantage Cons - Not quite as fast as RAID 5, "wastes" the storage space of two drives.

RAID 10 or RAID 1 0 - Combination of RAID 1 & 0. Essentially two RAID 0 arrays mirroring each other. Pros - High speed & can sustain multiple failures as long as they don't happen to be mirrors of each other. Cons - Very Expensive, 2x number of hard disks for storage space needed. RAID 5 w/ hot spare vs. RAID 6 From a data protection & recovery perspective the clear choice is RAID 6. You'd be amazed how often a second drive fails while rebuilding the hot spare, leaving the array broken. With six you can afford two simultaneous losses and still rebuild. There are some performance disadvantages of six but if you have a good controller card, you won't notice much impact.

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