RAID( Redundant Array of Independent Disks )

RAID is a data storage technology which can be configured for speed, for duplicate of data(mirroring) or can be use for protected storage, by arranging multiple disks in various arrangements. Basically used for data loss prevention.

However RAID is different from backup. RAID is storage protection OTG (on-the-go), disks in RAID system can be hot swapped( replace damaged disk in the RAID system without shutting down). Whereas backup is just copying data to another disk.

RAID can be categorize into Software RAID (SR) or Hardware RAID (HR). SR uses the operating system resources in which they are installed into. HR uses a dedicated controller.

The most common levels of RAID are RAID 0, RAID 1, RAID 5 and RAID 6.

RAID 0 (striping of data):

  • minimum 2 disks are used.
  • The data is strip and fills the buffer of individual disk. For example, RAID 0 with 3 disks. Data is feed into disk 1 buffer follow by disk 2 and disk 3. After which the cycle repeats.
  • This arrangement enables very fast read/ write.
  • Therfore RAID 0 is used when speed is important.
  • No redundancy, no parity and no mirroring.
  • No data loss prevention function.
  • Only reason to use RAID 0 is for speed.

RAID 1 (data mirroring):

  • minimum 2 disks are used.
  • Basic arrangement is 2 disks. When data is transferred to disk 1, they are also copied to disk 2 (disk mirroring).
  • More expensive arrangement as 2 disks are needed to store 1 disk data.
  • Used when speed is not a concern.
  • No parity.

RAID 5 (protected storage):

  • Minimum 3 disks are required.
  • Data is strip and distributed evenly among disks, just like RAID 0.
  • Parity bit (bit comparison) between any 2 disks is stored in the 3rd disk hence every disk hold parity information of another 2 disks. Aka distributed parity.
  • Because parity between any 2 disks is recorded, RAID 5 system is protected for 1 disk failure. This means if any 1 disk fails, the failed disk can be replaced with a new disk. The lost data will be built with the other 2 disks, following the parity bits information.

Ok, before we go to RAID 6, we will explain in detail how parity in RAID 5 works.

Disk 3 store parity bits of disk 1 & 2, disk 2 store parity bits of disk 1 & 3 and disk 1 store parity bits of disk 2 & 3.

RAID 6 ( similar to Raid 5 but with dual parity)

  • Minimum 4 disks are required.
  • Similar to Raid 5 but with dual parity.
  • Protected for 2 disks failure.