Q19 ) (Input / Output) A RAID can fail only if the data is not recoverable after a crash. Suppose that the probability of one drive crashing in a given hour is p. Also suppose that the RAID has 4 disk drives available for it.

What is the probability of a 4-drive RAID failing in a given hour if the used RAID is a: (Also make a single sentence notification on the amount of data lost in the event of a crash and possiblity of recovery; (For example, all disk lost, or one strip lost etc ).

a) RAID 0:

b) RAID 1

c) RAID 2

d) RAID 3 (one disk used as parity drive)

20  One way to use the contiguous allocation of the disk and not suffer from holes is to compact (defragment) the disk every time a file is removed. Since all files are contiguous, copying a file requires a disk seek followed by a rotational delay to read the file, followed by the transfer at full speed. Writing the file back requires the same work. Assuming:

• a seek time of 5 msec,
• a rotational delay of 5 msec,
• a transfer rate of 100 MB/sec,
• and average file size of 8KB,

(!!! msec = milliseconds, 10^-3 seconds!!)

a) How long does it take to read an average-sized file into the main memory and then write it back to the disk at a new location?

 b) Using these numbers, how long would it take to make a second copy of a 1-GB file assuming that the empty space of the disk is not fragmented? ( You can assume that file is written sequentially to the disk)

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