Networking (Storage) interview questions contributed by Jai(series 2)

Continuing the previous posting here some more interview questions contributed by our blog reader Jai.
More than 20 real storage job interview questions were posted in the previous post.

21) What is the purpose of disk array?
Probability of unavailability of data stored on the disk array due to single point failure is totally eliminated.

22) What is disk array?
Set of high performance storage disks that can store several terabytes of data. Single disk array can support multiple points of connection to the network.

23) What is virtualization?
A technique of hiding the physical characteristics of computer resources from the way in which other system application or end user interact with those resources. Aggregation, spanning or concatenation of the combined multiple resources into larger resource pools.

24) What is Multipath I/O?
Fault tolerant technique where by there is more than one physical path between the CPU in the computer systems and its main storage devices through the buses, controllers, switches and other bridge devices connecting them.

25) What is RAID?
Technology that groups several physical drives in a computer into an array that you can define as one or more logical drive. Each logical drive appears to the operating system as single drive. This grouping enhances the performance of the logical drive beyond the physical capability of the drives.

26) What is stripe-unit-size?
It is data distribution scheme that complement s the way operating system request data. Granularity at which data is stored on one drive of the array before subsequent data is stored on the next drive of the array. Stripe unit size should be close to the size of the system I/O request.

27) What is LUN Masking?
A method used to create an exclusive storage area and access control. And this can be achieved bye storage device control program.

28) What is the smallest unit of information transfer in FC?
Frame

29) How is the capacity of the HDD calculated?
Number of Heads X Number of Cylinders X Sectors per track X Sector Size

30) What is bad block reallocation?
A bad sector is remapped or reallocated to good spare block and this information is stored in the internal table on the hard disk drive. The bad blocks are identified during the media test of the HDD as well as during various types of read write operations performed during the I/O tests. Apart from the new generation of HDD comes with a technology called BGMS (background media scan) which continuously scans the HDD media for defects and maps them when the drive is idle (this is performed after the HDD is attached to the system).