RAID levels
There are a number of different RAID levels:

RAID 0
Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.

• Minimum number of drives: 2
• Strengths: Highest performance.
• Weaknesses: No data protection; One drive fails, all data is lost.

RAID 1 - Disk mirroring.

Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.

• Minimum number of drives: 2
• Strengths: Very high performance; Very high data protection; Very minimal penalty on write performance.
• Weaknesses: High redundancy cost overhead; because all data is duplicated, twice the storage capacity is required.

RAID 5 - Block-level data striping with distributed parity.
Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.

• Minimum number of drives: 3
• Strengths: Best cost/performance for transaction-oriented networks; Very high performance, very high data protection; Supports multiple simultaneous reads and writes; can also be optimized for large, sequential requests.
• Weaknesses: Write performance is slower than RAID 0 or RAID 1.
  

RAID - 0＋1
Combination of RAID 0 (data striping) and RAID 1 (mirroring).
Note that RAID 10 is another name for RAID (0+1) or RAID 0/1.
A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.

• 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.

From RAID1 to RAID5, whenever disk is damaged, we can extract the damaged and insert a new one (this needs hot-plugging in hardware), data will not be spoiled and the content of damaged disk can be reconstructed quickly. The reconstruction also can be completed by RAID hardware or RAID software. But RAID0 does not provide the error checking function, so someone said that it can not be regarded as RAID. Actually, that is why RAID0 is called RAID0, 0 represents "none".

Choice of RAID
There are three factors that may influence your choice of RAID level: usability (redundant data), performance and cost. If you do not need usability, RAID-0 can perform best. If usability and performance are more important than cost, RAID-1 or RAID-10(it depends on the number of disks) is a good choice. If price is as important as usability and performance, you can choose RAID-3,RAID-30,RAID-5 or RAID-50(it depends on the transmission type and the number of disk drivers)

Strengths of RAID:

Low cost, little power consumption, high transmission speed.
Fault-tolerance function.
Data security.

RAID applies for:

E-Mail server.
Workgroup/file server.
Corporation server.
Storage LAN.
Internet news server.

RAID failures
Nowadays, RAID is a quite good storage technique. It has advantages in storage capacity, storage security and storage speed. However, once there are some failures in RAID, the loss is ususally? tremendous.

Situations in which RAID information may lose:
The RAID array card breaks down
Physical failures of disk
Power cut
Extracting disk in wrong order
Reallocation RAID array information
Disk electric line falls off
High temperature of node caused by heat dissipation
Unstable voltage

Some methods for above questions
Not easily attempt rebuild, synchronization etc.
Not initialize
Mirror disk
Analyze mirror file, reconstruct data
Create RAID information, analyze the construction of data

Some problem can be solved by software, such as RAID Reconstrustor, R－Studio and Data Recovery Wizard Professional etc. These software can help recreate Raid information, analyze disk order, reconstruct array information, finally restoring Raid disk.

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