RAID 2
RAID是“Redundant Array of Independent Disk”的缩写，中文意思是独立冗余磁盘阵列。冗余磁盘阵列技术诞生于1987年，由美国加州大学伯克利分校提出。简单地解释，就是将N台硬盘通过RAID Controller（分Hardware，Software）结合成虚拟单台大容量的硬盘使用。RAID的采用为存储系统（或者服务器的内置存储）带来巨大利益，其中提高传输速率和提供容错功能是最大的优点。
RAID 2是RAID 0的改良版，以汉明码（Hamming Code）的方式将数据进行编码后分割为独立的位元，并将数据分别写入硬盘中。因为在数据中加入了错误修正码（ECC，Error Correction Code），所以数据整体的容量会比原始数据大一些。
The main problem with RAID-1 is that it is costly:You need twice as many disks to store a given amount of data.
RAID-2 takes the idea of data striping to the extreme.Instead of writing data in blocks of arbitary size,RAID-2 writes one bit per strip(as shown in the Figure).This requires a minimum of eight surfaces just to accommodate the data.Additional drives are used for error-correction information generated using a Hamming code. The number of Hamming code drives needed to correct single-bit errors is proportionate to the log of the number of data drives to be protected.If any one of the drives in the array fails,the Hamming code words can be used to reconstruct the failed drive.(Obviously,the Hamming drive can be reconstructed using the data drives.)
Because one bit is written per drive,the entire RAID-2 disk set acts as though it were one large data disk.The total amount of available storage is the sum of the storage capacities of the data drives.All of the drives -including the Hamming drives- must be synchronized exactly, otherwise the data becomes scrambled and the Hamming drives do no good.Hamming code generation is time-consuming;thus RAID-2 is too slow for most commercial implementations. In fact, most hard drives today have bulit-in CRC error correction.RAID-2,however,forms the theoretical bridge between RAID-1 and RAID-3, both of which are used in the real world.
Reference:<<The essentials of Computer Organization and Architecture>> Linda Null ,Julia Lobur
A RAID Level 2 configuration consists of a set of data drives, and a set of Hamming code drives.
– Hamming code drives provide error correction for the data drives.
– RAID 2 performance is poor and the cost is relatively high.
