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* Description: Knowledge of RAID hard disk level defination
* Compatiablity: RDBMS 11g, 12c
* Date: 02:12 PM EST, 04/03/2017
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<1> Disk Striping: 
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     |__ Defination: Disk striping is the process of dividing a body of data into blocks and spreading the data blocks across multiple storage devices.
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     |__ Advantage: The main advantage of disk striping is higher performance. 
     |              For example, striping data across three hard disks would provide three times the bandwidth of a single drive. 
     |              If each drive runs at 200 input/output operations per second (IOPS), disk striping would make available up to 600 IOPS for data reads and writes.
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     |__ Disadvantage: Disk striping is low resiliency. The failure of any physical drive in the striped disk set results in the loss of the data on the striped unit, 
     |                 and consequently, the loss of the entire data set stored across the set of striped hard disks.
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     |__ Disk striping and RAID: Redundant array of independent disks (RAID) uses disk striping to distribute and store data across multiple physical drives. 
     |                           Disk striping is synonymous with RAID 0 and spreads the data across all the disk drives in a RAID group without parity. 
     |                           Disk striping without parity is not fault tolerant.	 
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     |__ Summary: RAID - Redundency Array of Indenpendent Disk, simply means combining several seperated physical disks into one logical "Disk", which makes the data 
                  more safe [mirror backup feature] and I/O more fast [striped disk].	 
	 
				
<2> Disk Level:					   
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     |__ RAID  0 - used to boost a server's performance. It's also known as "disk striping." With RAID 0, data is written across multiple disks. 
     |             This means the work that the computer is doing is handled by multiple disks rather than just one, increasing performance 
     |             because multiple drives are reading and writing data, improving disk I/O. A minimum of two disks is required. Both software and hardware RAID support RAID 0, 
     |             as do most controllers. The downside is that there is no fault tolerance. If one disk fails, then that affects the entire array and the chances for data loss 
     |             or corruption increases.
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     |__ RAID  1 - a fault-tolerance configuration known as "disk mirroring." With RAID 1, data is copied seamlessly and simultaneously, from one disk to another, creating a replica, 
     |             or mirror. If one disk gets fried, the other can keep working. It's the simplest way to implement fault tolerance and it's relatively low cost.
     |             The downside is that RAID 1 causes a slight drag on performance. A minimum of two disks is required for RAID 1 hardware implementations. 
     |             One additional point - If a server with two 1TB drives is configured with RAID 1, then total storage capacity will be 1TB not 2TB.	 
     |         
     |__ RAID  5 - By far the most common RAID configuration for business servers and enterprise NAS devices. 
     |             This RAID level provides better performance than mirroring as well as fault tolerance. With RAID 5, data and parity (which is additional data used for recovery) 
     |             are striped across three or more disks. If a disk gets an error or starts to fail, data is recreated from this distributed data and parity block— seamlessly 
     |             and automatically. Essentially, the system is still operational even when one disk kicks the bucket and until you can replace the failed drive. 
     |             Another benefit of RAID 5 is that it allows many NAS and server drives to be "hot-swappable" meaning in case a drive in the array fails, that drive can be swapped 
     |             with a new drive without shutting down the server or NAS and without having to interrupt users who may be accessing the server or NAS.
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     |             >> [At least 3 disks are required] For example 3 disks in RAID 5 group, 2 disks will be written with data bytes, the rest one will be recorded with "Parity" info.
     |                Through algorathm, the 3 bytes info will be distributed into 3 different disks. If any one disk got corrupted, the data could be recovered by rest 2 via XOR. 
     |                Parity - 奇偶校验位， A B 两位只有相同为真，不同为假。	 
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     |__ RAID 10 - A combination of RAID 1 and 0 and is often denoted as RAID 1+0. It combines the mirroring of RAID 1 with the striping of RAID 0. 
                   It's the RAID level that gives the best performance, but it is also costly, requiring twice as many disks as other RAID levels, for a minimum of four. 
                   This is the RAID level ideal for highly utilized database servers or any server that's performing many write operations.	 
				   
				   
Reference: 
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     |__ http://www.pcmag.com/article2/0,2817,2370235,00.asp
         http://www.eng.famu.fsu.edu/~tsai/raid/raid.html