What is Raid?

The basic designing element of RAID is to assemble multiple inexpensive hard drives to create a hard drive set in order to exceed a single, expensive hard drive. According to different version of RAID, RAID has more advantages than single hard drive, such as enforce data integration, and reinforce fault tolerance function, and increase transfer speed or storage.

With RAID enabled on a storage system you can connect two or more drives in the system so that they act like one big fast drive or
set them up so that one drive in the system is used to automatically and instantaneously duplicate (or mirror) your data for real-time backup.

Moreover, RAID hard drive array can be seen as a single hard drive or logistic storage unit, which can be catalogued in RAID-0, RAID-1, RAID-5, and RAID-10. RAID can be classified in various levels, which has their advantages and pitfalls respectively, user can select different level according their various requirements.

In the event of a drive failure (fault-tolerant RAID modes only), data is still available in its entirety to users. However, the portion of the requested data that resides on the failed drive needs to be re-created from the parity information on the healthy drives(s). Therefore, if a drive failure is present, the faulty drive should be replaced as soon as possible. Keep in mind that in the event of a drive failure, there is no more additional redundancy, so if a second drive fails, data loss is often avoidable. There are specific RAID modes and other similar technologies such as "Hot-spare" (link to Hot-spare) that assist in providing additional layers of protection.

After replacing the failed drive, the RAID architecture is rebuilt, restoring redundancy to the array.

The term and technology RAID has been safely used by the world's largest companies since 1987. For nearly the past 10 years, RAID options have been commonplace in small business and consumer applications.

Raid 0

Set in high-performance mode (also called striped mode or RAID 0) the storage system gives you the power you need when you’re:

Designing huge graphics and need a lightning-fast Photoshop® scratch space.
Recording large DV fles while maintaining clean audio performance.
Editing DV or HD video and want a smooth work fow with no dropped frames.
Rendering complex 3D objects or special effects.
Performing disk-intensive database operations.
Driven to be the frst geek on your block with a computer so fast it blows your socks off.

Data is saved (striped) across the drives and accessed in parallel by all the drives so you get higher data transfer rates on large data accesses and higher input/output rates on small data accesses.

Below table lists values used in the Primary_RAID_Level field of the Virtual Disk Configuration Record and the definitions of these values. The table defines the standard RAID levels, such as RAID 0, 1, 3, 5, etc. and some proprietary RAID types. Non-RAID types such as JBOD and concatenation are also included for completeness.

Name	PRL Byte	Description
RAID-0	00	Striped array with no parity
RAID-1	01	Mirrored array
RAID-3	03	Striped array with typically non-rotating parity, optimized for long, single-threaded transfers
RAID-4	04	Striped array with typically non-rotating parity, optimized for short, multi-threaded transfers
RAID-5	05	Striped array with typically rotating parity, optimized for short, multi- threaded transfers
RAID-6	06	Similar to RAID-5, but with dual rotating parity physical disks, tolerating two physical disk failures
RAID-1E	11	>2 disk RAID-1, similar to RAID-10 but with striping integrated into array
Single Disk	0F	Single, non-arrayed disk
Concatenation	1F	Physical disks combined head to tail
RAID-5E	15	RAID-5 with hot space at end of array
RAID-5EE	25	RAID-5 with hot space integrated into array

Raid 1

Set the system to data protection mode (also known as mirrored mode or RAID 1) and the capacity is divided in half. Half of the capacity is used to store your data and half is used for a duplicate copy.

Raid 5

In systems with three or more drives, we recommend that you set the system to RAID 5. This gives you the best of both worlds: fast
performance by striping data across all drives; data protection by dedicating a quarter of each drive to fault tolerance leaving three quarters of the system capacity available for data storage.

Raid 10

RAID-10 and RAID-01 are not the same thing and it does matter. RAID-01 is a mirrored configuration of two striped sets. RAID-10 is a stripe across a number of mirrored sets.

RAID-10 provides better fault resilience and "rebuild" performance than RAID-01. Both array types provide very good to excellent overall performance by combining the speed of RAID-0 with the redundancy of RAID-1 without requiring parity calculations.

Minimum number of drives: 4

Strengths: Highest performance, highest data protection (can tolerate multiple drive failures).

Weaknesses: High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required; Requires minimum of four drives.