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.
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.
Advantages and disadvantages of various level of RAID
Name
Description of RAID
and required hard drive amounts
Data Reliability
Data Transfer Efficiency
Maximum I/O Transferring Rate
RAID 0

Striping
Data parallel storage without fault tolerance function.
Minimum 2 hard drives, the storage amount is accumulated.

Lower than single hard drive.

Highest.

Writing and accessing speed are both very high.

RAID 1

Mirroring

All data will be duplicated while accessing RAID 1.

Highest.

Accessing speed is higher than single hard drive, writing speed is similar to single hard drive.

Maximum accessing speed can reach twice of single speed, writing speed is similar to single hard drive.

RAID 5

Striped
with Rotating parity

Data and parity check will be recorded in different hard drive. Minimum 3 hard drives.

Much higher than RAID 0, and same as RAID 1, RAID 10.

Accessing speed is similar to RAID 0, writing speed is lower than single hard drive.

Accessing speed is close to RAID 0, writing speed is lower than single hard drive.

RAID 10

Mirror Stripe

Includes advantages from both RAID 0 and RAID 1.
Required hard drives amount is twice of RAID 0 or RAID 1.

Highest.

Highest.

Both accessing and writing speed are very high.

 
RAID 0
There should be at least two hard drives to create RAID 0 without protection. RAID 0 is combining two or more hard drives with data accessing simultaneously from both drives with the transferring speed higher than any other single hard drive. RAID 0 can function with high data transferring ratio, but without backup for data. Once there an error occurs in any of the hard drives it will influence all data, therefore RAID 0 is not suitable for high requirement of data security.

RAID total storage is the sum of all hard drives.

Example:Data will be divided into data 1 and data 2, writing into hard drive A and B respectively in order to increase accessing speed.
 
 
RAID 1
RAID 1 is consisted of two hard drives into one set, every data will be written on two hard drives identically and RAID 1 will only present one single hard drive in the system. Because RAID 1 enables two hard drives to be accessed at the same time, therefore, the data writing speed might be slower while the accessing speed is unaffected. The utility rate of RAID 1 is the lowest of all RAID level while the unit cost is the highest. But RAID 1 provides high data security when one of the hard drives is out of order, the system can switch to mirroring hard drive to access data without defragging error data
RAID total storage is based on the smallest hard drives in the array.
Example:The same data is writing into hard drive A and B respectively.
 
 
RAID 5
To assemble a RAID 5, a minimum of three hard drives are required. RAID 5 is not a support backup for data’s storage, instead it writes the data and corresponding parity check onto different hard drives. If an error occurs in one of the hard drives, RAID 5 can use the remaining data and corresponding parity check to recover the damaged data. RAID 5 can be seen as a solution with best storage function, safety data, and cost efficiency. RAID 5 can provide high speed data access for multi user, and the total storage amount of RAID 5 is the total hard drive storage minus one hard drive in the array.
Example:RAID 5 and RAID 0 have similar data accessing speed, but RAID 5 includes parity check and the writing data speed is slower than single hard drive. Moreover, due to multi data correspondence to parity check, the usage ratio of RAID 5 storage is higher than RAID 1, and therefore the storage cost is lower than RAID 1.
 
 
RAID 10
To assemble RAID 10, a minimum four hard drives are required and half of the total hard drives storage are used for mirroring storage, therefore, RAID 10 can be seen as a highest unit cost of RAID modules. RAID 10 is only suitable for high efficiency, high fault tolerance, but with the low request condition. Raid 10 is the combination of RAID 1 and RAID 0, in RAID 10 module the system reads and writes similar to a RAID 1 array that is further split into RAID 0.
Example:The original data is “AB”, when writing into RAID 10 the data will be divided into A and B, then backup to A1, A2 and B1, B2. The data of A1 and A2 are identical, while B1 and B2 are identical too. Therefore, under the condition that two hard drives are out of order, the original data “AB” can be recovered if data A1 and A2 are not out of the order at the same time or B1 and B2 are not out of the order at the same time.