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Lesson 9 RAID levels
Objective Describe RAID levels.

Describe RAID levels and Data Loss Protection

Standard RAID consists of levels from 0-5. Each level represents a different application of the technology. For example:
  1. Level 1 corresponds to mirroring. Mirroring uses n partitions that all store the same data.
  2. Level 5 corresponds to striping with parity. Striping is the arrangement of contiguous disk blocks across multiple drives instead of within the same drive. The idea behind this arrangement is to increase the efficiency of disk access by allowing concurrent seeks on multiple devices. Parity is the calculation of an additional stripe from the normal data stripes, to be used to recover lost data in the event of a drive failure.

The following table compares and contrasts the various RAID levels.

Level Characteristics Advantages Disadvantages
Linear Concatenates small partitions into a larger one Allows two disks to be used linearly as one, with the capacity of subsequent disks simply added to the first disk No redundancy; less reliable overall because if one partition fails, the larger one fails
RAID 0 Striping Performance benefits for reads and writes No fault tolerance
RAID 1 Mirroring Efficient for reads because multiple reads can be done on multiple drives; complete fault tolerance Diminished write performance, because of the need to write data more than once; high cost per megabyte of storage due to the duplication of all data
RAID 2 not used
RAID 3 not used
RAID 4 Striping, parity, dedicated parity disk Fault tolerance without the high cost of completely duplicating all data Parity disk can become a performance bottleneck; for this reason it is almost never used over RAID 5
RAID 5 Striping, parity Fault tolerance without the high cost of completely duplicating all data  

The following SlideShow illustrates the various methods RAID uses to store data.
  1. Writes and reads occur simultaneously on each drive
  2. Duplicate data is written to pairs of drives. Reads can occur simultaneously on every drive.
  3. Every write must update the dedicated parity drive. Reads can occur simultaneously on every data drive
  4. Writes require parity to be updated. Reads can occur simultaneously on every data drive.

Read Write Raid Levels
The next lesson compares and contrasts hardware and software RAID.

Raid Levels - Quiz

Before moving on to the next lesson, click the Quiz link below to check your understanding of RAID levels.
Raid Levels - Quiz