Shulou(Shulou.com)06/03 Report--

RAID 5 is a storage solution that combines storage performance, data security, and storage costs. RAID 5 can be understood as a compromise between RAID 0 and RAID 1. RAID 5 can provide data security for the system, but the degree of protection is lower than that of Mirror and the utilization of disk space is higher than Mirror. RAID 5 has a data read speed similar to that of RAID 0, but with one more parity information, and writing data is slightly slower than writing to a single disk. At the same time, because multiple data correspond to a parity information, the disk space utilization of RAID 5 is higher than that of RAID 1, and the storage cost is relatively low.

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RAID5E is a high level of RAID disk storage. RAID5E (RAID5 Enhancement) is an improvement based on the RAID5 level. Similar to RAID5, the check information of the data is evenly distributed on each hard disk, but a part of unused space is reserved on each hard disk. This part of the space is not striped, allowing up to two physical hard drives to fail. It seems that RAID 5e and RAID5 plus a hot spare are similar, but because RAID 5e distributes data on all hard drives, its performance will be better than that of RAID5 plus a hot spare. When a hard disk fails, the data on the failed hard disk will be compressed to unused space on other hard drives, and the logical disk remains at the RAID 5 level.

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RAID 5EE provides a perfect alternative to the "RAID5+HotSpare" disk. The original single HotSpare hot spare was also striped by Stripe and evenly distributed among 5 disks. In this way, when reading and writing to RAID 5EE, five disks participate in Icano at the same time, which increases the read and write bandwidth of one disk and improves the performance compared with the case of four disks + HotSpare disks. RAID 5EE's data distribution is more efficient, and a portion of the space on each hard disk is used as a distributed hot spare, which is part of the array, and data is rebuilt faster when a physical hard disk in the array fails. RAID5EE can work when two disks in the disk array are dropped at the same time, while RAID5 can only work when at most one disk is offline.

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Generally speaking, raid5e,raid5ee adds the parallel read and write function of disk on the basis of raid5. Moreover, raid5ee supports two disconnected disks to continue to work, which is more stable than raid5.

The data recovery principle of riad5,raid5e,raid5ee is based on parity, P=D1 xor D2 xor D3. Xor Dn (D1, D2, D3... Dn is a data block, P is a check, and xor is an XOR operation)

The definition of the XOR operation between two numbers is:

XOR 1 = 0

1 XOR 0 = 1

0 XOR 1 = 1

0 XOR 0 = 0

When there are multiple digital XOR, there are two characteristics:

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A) the result is independent of the order of operation. That is, (a XOR b) XOR c = a XOR (b XOR c).

B) the numbers involved in the operation are circularly symmetrical with the results. If a XOR b XOR c = d, then a = b XOR c XOR d * b = a XOR c XOR D * * C = a XOR b XOR d.

The reason why RAID5 in disk array is fault-tolerant is that it takes advantage of these characteristics of XOR operation. In the above example, a, b, c, d can be thought of as data on four disks, three of which are application data and the remaining one is parity. When you encounter a fault, no matter which one you can't find, you can use the remaining three numbers XOR to figure it out. In practical application, the array controller generally divides the disk into many stripes (Stripe, note that it is not Stripper), and then XOR each set of stripes.

Understand the principle of RAID5, the principle of RAID5 data recovery is not difficult to understand. After the array is lost, the data recovery engineer looks at the existing information of the hard disk, through calculation, finds out the original disk order, block size and other relevant information, and manually simulates the original state of creating the array, thus reading out all the data. Therefore, if the Rebuild reorganization operation is performed after the loss of RAID5 data, the original array information will no longer exist, the original parameters cannot be calculated, and the data will not be recovered; on the contrary, if Rebuild reorganization and other related operations are not carried out, RAID5 data recovery can almost achieve 100% success rate.

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