Compute the average time to read or write a 2048-byte sector of disk rotating at

Question

Compute the average time to read or write a 2048-byte sector of disk rotating at 14, 000 RPM, given that the seek time is 3ms, data transfer rate is 20 MB/sec, and controller overhead is 0.2 ms. Assume there are no pending jobs on the disk. Discuss with the help of necessary figures the disk storage systems RA1D0 through to RAID6 and outline the differences between them with particular emphasis on the write update schemes of RAID-3 and RAID-4, and advantageous scenarios for RAID-5 vis-a-vis RAID-4.

Explanation / Answer

Answer to 3.a-

Average Tseek=3ms

Average Trot=0.5*60/14,000=2.14ms

Ttransfer=1KB/20MB=0.02ms

Tcontroller=0.2ms

The total time taken= Tseek +Trot+ Ttsfr +Tctr=

Answer to 3.b-

RAID is a technology that is used to increase the performance and/or reliability of data storage. The abbreviation stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more drives working in parallel. These disks can be hard discs, but there is a trend to also use the technology for SSD (solid state drives). There are different RAID levels.

Striping

RAID is able to offer increased performance by using a data storage technique called striping. Data striping organizes the data on your hard disks in a way that allows for faster data access.

Mirroring

RAID offers increased data protection by using a data storage technique called mirroring. In mirroring, the data on your hard disks is replicated thereby producing data redundancy across your storage volume. This ensures greater protection for your data.

RAID 0- It consists of striping, without mirroring or parity. The capacity of a RAID 0 volume is the sum of the capacities of the disks in the set, the same as with a spanned volume. There is no added redundancy for handling disk failures, just as with a spanned volume. Thus, failure of one disk causes the loss of the entire RAID 0volume, with reduced possibilities of data recovery when compared to a broken spanned volume. Striping distributes the contents of files roughly equally among all disks in the set, which makes concurrent read or write operations on the multiple disks almost inevitable and results in performance improvements.

RAID 1- It consists of data mirroring, without parity or striping. Data is written identically to two (or more) drives, thereby producing a "mirrored set" of drives. Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first (depending on its seek time and rotational latency), improving performance. Sustained read throughput, if the controller or software is optimized for it, approaches the sum of throughputs of every drive in the set, just as for RAID 0. Actual read throughput of most RAID 1 implementations is slower than the fastest drive. Write throughput is always slower because every drive must be updated, and the slowest drive limits the write performance.

RAID 2- It consists of bit-level striping with dedicated Hamming-code parity. All disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive. This level is of historical significance only; although it was used on some early machines.

RAID 3- It consists of byte-level striping with dedicated parity. All disk spindle rotation is synchronized and data is striped such that each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.

RAID 4 – It consists of block-level striping with dedicated parity. The main advantage of RAID 4 over RAID 2 and 3 is I/O parallelism: in RAID 2 and 3, a single read/write I/O operation requires reading the whole group of data drives, while in RAID 4 one I/O read/write operation does not have to spread across all data drives. As a result, more I/O operations can be executed in parallel, improving the performance of small transfers.

RAID 5- It consists of block-level striping with distributed parity. Unlike RAID 4, parity information is distributed among the drives, requiring all drives but one to be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.[11] RAID 5 is seriously affected by the general trends regarding array rebuild time and the chance of drive failure during rebuild. Rebuilding an array requires reading all data from all disks, opening a chance for a second drive failure and the loss of the entire array.

RAID 6- It consists of block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore. RAID 6 requires a minimum of four disks. As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced.[11] With a RAID 6 array, using drives from multiple sources and manufacturers, it is possible to mitigate most of the problems associated with RAID 5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID 6 instead of RAID 5.

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