RAID
Redundant Array of Independent Disks, a collection of two or more disks working together in an array. Mylex RAID controllers implement this technology to connect up to 15 SCSI devices per channel. The different forms of RAID implementation are known as “RAID levels.” See also Berkeley RAID Levels, Disk Array, and RAID Levels.
The system manager or integrator selects the appropriate RAID level for a system. This decision will be based on which of the following are to be emphasized:
- Disk Capacity
- Data Availability (redundancy or fault tolerance)
- Disk Performance
RAID Adapters
See RAID Controller
RAID Advisory Board (RAB)
An association of companies whose primary intention is to standardize RAID storage systems. Mylex is a member of RAB.
RAID Controller
Low cost RAID controllers that use SCSI channels on the motherboard.
RAID Levels
Mylex disk array controllers support four RAID Advisory Board approved (RAID 0, RAID 1, RAID 3, and RAID 5), two special (RAID 0+1, and JBOD), and three spanned (RAID 10, 30, and 50) RAID levels. All DAC960, AcceleRAID, and eXtremeRAID series controllers support these RAID levels. See also Berkeley RAID Levels.
– Level 0:
Provides block “striping” across multiple drives, yielding higher performance than is possible with individual drives. This level does not provide any redundancy.
– Level 1:
Drives are paired and mirrored. All data is 100 percent duplicated on a drive of equivalent size.
– Level 3:
Data is “striped” across several physical drives. Maintains parity information, which can be used for data recovery.
– Level 5:
Data is “striped” across several physical drives. For data redundancy, drives are encoded with rotated XOR redundancy.
– Level 0+1:
Combines RAID 0 striping and RAID 1 mirroring. This level provides redundancy through mirroring.
– JBOD:
Sometimes referred to as “Just a Bunch of Drives.” Each drive is operated independently like a normal disk controller, or drives may be spanned and seen as a single drive. This level does not provide data redundancy.
– Level 10:
Combines RAID 0 striping and RAID 1 mirroring spanned across multiple drive groups (super drive group). This level provides redundancy through mirroring and better performance than Level 1 alone.
– Level 30:
Data is “striped” across multiple drive groups (super drive group). Maintains parity information, which can be used for data recovery.
– Level 50:
Data is “striped” across multiple drive groups (super drive group). For data redundancy, drives are encoded with rotated XOR redundancy.
Note: The host operating system drivers and software utilities remain unchanged regardless of the level of RAID installed. The controller makes the physical configuration and RAID level implementation.
RAID Migration
A feature in RAID subsystems that allows for changing a RAID level to another level without powering down the system.
Read-Ahead Cache
A caching strategy whereby the computer anticipates data and holds it in cache until requested.
Recovery
The process of reconstructing data from a failed disk using data from other drives.
Redundancy
The inclusion of extra components of a given type in a system (beyond those the system requires to carry out its functions).
Rotated XOR Redundancy
XOR refers to the Boolean “Exclusive-OR” operator. Also known as Parity, a method of providing complete data redundancy while requiring only a fraction of the storage capacity of mirroring. In a system configured under RAID 3 or RAID 5 (which require at least three SCSI drives), all data and parity blocks are divided amongst the drives in such a way that if any single drive is removed (or fails), the data on it can be reconstructed using the data on the remaining drives. In any RAID 3 or RAID 5 array, the capacity allocated to redundancy is the equivalent of one drive.