EXTERNAL MEMORY
RAID LEVEL 6
RAID 6 schemes have two different parity calculations which are carried out and stored in separate blocks on different disks, so that a RAID 6 array which user data require N disks
Advantage of RAID 6 is provides extremely high data availability.
RAID LEVEL 5
RAID Level 5 is similar to RAID 4. The difference is RAID 5 distributes the parity strips across all disks. The distribution of parity strips across all drives avoids the potential I/O bottleneck which found in RAID 4.
RAID Level 4
RAID 4 makes use of an independent access array which each member disk operates independently. So separate I/O request can be satisfied in parallel that makes independent access arrays are more suitable for applications that require high I/O request rates but it’s not suitable for the application that require high data transfer rates.
Data striping is used in RAID 4 which a bit-by-bit parity strip is calculates across the same strips on each data disks and the parity bits are stored in the same strip on the parity disk.
Every write operation must involve the parity disk, which can become a bottleneck.
RAID LEVEL 3
RAID 3 is similar to RAID 2. The different is RAID 3 require only a single redundant disk, even if the disk array is large. RAID 3 employs parallel access with data distributed in small strips. From that reason, RAID 3 can achieve very high data transfer rates.
RAID Level 2
RAID 2 makes use of parallel access technique. In RAID 2 the strip is very small so an error-correcting code is stored in the same bit positions on multiple parity disks. It uses Hamming code to correct single-bit error and detect double-bit error.
RAID 2 is still costly because the number of redundant disks is proportional to do the log of the number of data disks. On a single read, if there is a single-bit error, controller can recognize and correct the error immediately so that the read access time is not slowed. On a single write, all data disks and parity disks must be accessed for the write operation.
RAID LEVEL 1
RAID 1 have different thing from RAID 2 through RAID 6 that is redundancy is achieved. Redundancy in RAID 1 is achieved by the simple expedient of duplicating all the data. From the picture below, data striping is like in RAID 0 but each logical strip in RAID 1 is mapped to two separate physical disks. So every disk in RAID 1 have a mirror disk which contains the same data.
​
Advantage of using RAID 1
A read request can be serviced by either of two disks that contains the request data which selected by less seek time and less rotational latency.
A write request require the same updated strips, but this can be done in parallel.
Because of parallel drives, the problem of drive fails is done.
Disadvantage of using RAID 1
Cost
If one of disks is failed, it must be effect on all of data system
RAID Level 0
RAID level 0 is not a true member of the RAID family, because it does not include redundancy to improve performance.
​
RAID 0 for High data transfer Capacity
In RAID 0 and any of the RAID, performance of them depends on request patterns of host system and on layout of the data. For applications to see a high transfer rate must have two requirements. The first one is a high transfer capacity must exit along entire path between host, memory and disk drives and also include internal controller buses, host system I/O buses, I/O adapters and host memory buses. The second requirement is the application must make I/O request that drive the disk array efficiently.
RAID 0 for High I/O Request Rate
The performance of RAID also depends on strip size. If strip size is large, a single I/O request will involve with a single disk access, and multiple I/O request can be handled parallel, reducing the queuing time for each request.
RAID
With the use of multiple disk, there is a wide variety of way in which the data can be organized. This could make it difficult to develop database schemes. Industry has agreed on a standardized scheme for multiple-disk database design. The RAID scheme consist of seven levels
These levels imply to designate different design architecture that share three common characteristic
1. RAID is asset of physical disk drives viewed by the operating system as a single logical drive.
2. Data are distributed across the physical drives of an array
3. Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure
The details of second and third characteristics differ for the different RAID levels. RAID 0 does not support the third characteristic.