RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places (thus, redundantly) on multiple hard disks. By placing data on multiple disks, input/output operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failures, storing data redundantly also increases fault tolerance.
Advantages of RAID
There are three reasons you might want a RAID system of drives.
- You need tons of storage space and you need it to be fast. (RAID 0)
- You want to instantaneously and automatically backup your data. (RAID 1)
- You want both. (RAID 5)
Let’s discuss little bit about types of RAID:
1. Speed (RAID 0) Striped Disk Array with fault tolerance requires a minimum of 2 drives to implement.
Description: The simplest RAID level, RAID 0 should really be called "AID", since it involves no redundancy. Files are broken into stripes of a size dictated by the user-defined stripe size of the array, and stripes are sent to each disk in the array. Giving up redundancy allows this RAID level the best overall performance characteristics of the single RAID levels, especially for its cost. For this reason, it is becoming increasingly popular by performance-seekers, especially in the lower end of the marketplace.
Characteristics and Advantages:
- RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive.
- I/O performance is greatly improved by spreading the I/O load across many channels and drives.
- Best performance is achieved when data is striped across multiple controllers with only one drive per controller.
- No parity calculation overhead is involved.
- Very simple design.
- Easy to implement.
Disadvantages:
- Not a "True" RAID because it is NOT fault-tolerant
- The failure of just one drive will result in all data in an array being lost
- Should never be used in mission critical environments
2. Data protection (RAID 1) Mirroring and Duplexing requires a minimum of 2 drives to implement:
Description: RAID 1 is usually implemented as mirroring; a drive has its data duplicated on two different drives using either a hardware RAID controller or software (generally via the operating system). If either drive fails, the other continues to function as a single drive until the failed drive is replaced. Conceptually simple, RAID 1 is popular for those who require fault tolerance and don't need top-notch read performance. A variant of RAID 1 is duplexing, which duplicates the controller card as well as the drive, providing tolerance against failures of either a drive or a controller. It is much less commonly seen than straight mirroring
Characteristics and Advantages:
- One Write or two Reads possible per mirrored pair
- Twice the Read transaction rate of single disks, same Write transaction rate as single disks
- 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk
- Transfer rate per block is equal to that of a single disk
- Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures
- Simplest RAID storage subsystem design
Disadvantages:
- Highest disk overhead of all RAID types (100%) - inefficient
- Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended
- May not support hot swap of failed disk when implemented in "software".
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