RAID stands for Redundant Array of Inexpensive Disks. That means that RAID is a way of logically putting multiple disks together into a single array. The idea then is that these disks working together will have the speed and/or reliability of a more expensive disk. Now, the exact speed and reliability you’ll achieve from RAID depends on the type of RAID you’re using.
RAID storage is a complex area with plenty of technical terms to get your head around. We’ll explain the most common ones now as we’ll be using some of them later in this article.
- Parity: distributed information that allows the recreation of data stored within a RAID array, even if a disk fails.
- Mirroring: when data from one or more hard drives is copied onto another physical disk or disks.
- Striping: a method that involves writing data across multiple disks. In the example below, data is written across the drives in sequential order until it reaches the last drive. Then it jumps back to the first drive and starts a second stripe before repeating.
- Block: the logical space on each disk where the data is written. The amount of space is set by the RAID controller.
- Left/right symmetry: symmetry in a RAID array determines the way in which the data and parity are distributed across the drives. There are four main styles of symmetry that can be used (depending on the RAID vendor) and some companies make proprietary styles to meet their own business needs.
- Hot spare: a spare disk that can be used in place of a failed disk within a RAID array.
- Degraded mode: this occurs when a drive in the RAID becomes unreadable and is withdrawn from the array. The new data and parity are then written to the remaining drives within the RAID. If any data is requested from the failed drive, it is worked out with the parity on the others. The decrease in drives degrades the performance of the RAID, hence degraded mode.
Instead of explaining each RAID separately, I prefered adding this table that explains and shows precisely the difference between each RAID modes.
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