RAID (Redundant Array of Independent Disks) is a technology for managing how data is stored on the physical disks that reside in your system or are attached to it.
RAID is a method of combining several hard drives into one unit.
Ex: RAID has an ability to span physical disks so that the combined storage capacity of multiple physical disks can be treated as a single. The below diagram shows 3 physical HDD of size 900 GB, after configuring RAID (RAID 5), we can see a single volume of 1800 GB HDD.
Advantages of RAID:
- Increases the performance and reliability of data storage.
- Ability to maintain redundant data which can be used to restore data in the event of a disk failure
- In event failure, if one of the drives fails then either drive swapped out for a new drive without turning the systems off also known as hot swapable.
- Increase the parity check and regularly checks for any possible system crash.
- It offers fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives.
- Reading and Writing of data done at simultaneously.
- Disk Stripping makes multiple smaller hard disks to a single large volume.
- RAID cannot completely protect your data.
- RAID doesn't always result in improved system performance.
- Expensive, must purchase and maintain RAID controllers and dedicated hard drives.
- Need to update firmware for RAID controller’s regularly.
- If RAID controller fails, none of the HDD can be accessible, due to this server may go down.
RAID uses different techniques for writing data to disks. These techniques enable RAID to provide data redundancy or better performance. These techniques include
- Mirroring: Copying data from one physical disk to another physical disk. Mirroring provides data redundancy by maintaining two copies of the same data on different physical disks. If one of the disks in the mirror fails, the system can continue to operate using the unaffected (working) disk. Both sides of the mirror contain the same data at all times. Either side of the mirror can act as the operational side.
- Striping: Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual disk data addresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern. For example, if the virtual disk includes five physical disks, the stripe writes data to physical disks one through five without repeating any of the physical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion of a stripe that resides on a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in combination with parity does provide data redundancy.
- Parity: Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the striped disks fails, the data can be reconstructed from the parity information using the algorithm.
- Span: A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10 or 50 virtual disk.