I keep hearing these over and over but I never did quite get how it works since the number is different and it does different thing apparently and wikipedia is quite not explaining it’s benefits and downsides. So Could someone explain how these raid systems work.
In: Technology
RAID basically means using multiple hard drives to provide, primarily, data backup and recovery functionality. That’s the main benefit. The downsides are the cost of having additional hard drives and needing specialized hardware/software to implement RAID functionality.
RAID 0 basically takes your data and spreads it out evenly over multiple disks. This does not provide for backup and recovery, but can improve performance and allows the computer to access data faster. The downside is the loss of any single hard drive in the array makes the entire array unusual as your data is broken up across all of them.
RAID 1 is a simple mirroring of one hard drive to another. This provides the simplest form of backup and recovery. If one hard drive fails, you have an exact copy of it. But it isn’t very efficient.
RAID 2 is basically just a different implementation of RAID 0. RAID 0 distributes data across individual hard drives in clumps called “blocks” whereas RAID 2 does it on a bit-by-bit basis. However it does implement a simple form of error correction whereas RAID 0 does not.
RAID 3 is like RAID 0 and RAID 2, but it operates at the byte level (rather than block or bit) and one of the disks of the array is dedicated for *parity*. The parity disk examples the data on all the other disks and stores the parity of that data. For example, with RAID 3 we’re storing information byte-by-byte. Let’s say the first byte on the first disk is 01010101 and the first byte on the second disk is 00001111 and the first byte on the third disk is 00111100. The first byte of the parity disk looks at the first byte on all the other disks and counts how many “1”s appear at each bit place. It then stores a “0” if there are an even amount of “1”s and stores a “1” if there are an odd amount of “1”s like so:
|First Disk|01010101|
|:-|:-|
|Second Disk|00001111|
|Third Disk|00111100|
|Parity Disk|01100110|
Having a parity disk allows the recovery of a single hard drive. If a hard drive fails, you can construct what the missing byte would be by looking at the remaining bytes, the parity, then constructing the missing byte by using whatever bits are necessary to make the parity correct.
RAID 4 is like RAID 3 but operates on block level.
RAID 5 is like RAID 4 but the parity blocks are distributed across all of the hard drives in the array rather than being confined to one. The parity blocks only come into play when a disk fails, so having all of the parity blocks on a single hard drive means you have an entire hard drive that is hardly being used, increasing the wear and tear on the other hard drives. By distributing the parity blocks throughout, you include all the hard drives and distribute this stress more evenly.
RAID 6 extends RAID 5 by having two parity blocks per data block and therefore can support the loss of up to two hard drives.
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