Question

16.50. Suppose we have a sequential (ordered) file of 100,000 records where each record is 240 bytes. Assume that B = 2,400 bytes, s = 16 ms, rd = 8.3 ms, and btt = 0.8 ms. Suppose we want to make X independent random record reads from the file. We could make X random block reads or we could perform one exhaustive read of the entire file looking for those X records. The question is to decide when it would be more efficient to perform one exhaustive read of the entire file than to perform X individual random reads. That is, what is the value for X when an exhaustive read of the file is more efficient than random X reads? Develop this as a function of X.

Answer 1

Answer and Explanation:

Given that total number of records in a file = 100000

Each record consists of= 240 bytes

Given that B= 2400 bytes.

Then total blocks in file = 100000 records * 240 / 2400

= 10000 blocks.

Time that will take for exhaustive read

= s + r + b.btt

= 16 + 8.3 + (10000) * 0.8

= 8024.3 msec

Now as per given X be the number of records that are searched randomly and it takes more than exhaustive read time.

Hence, X (s + r + btt) > 8024.3

X (16+8.3+0.8) > 8024.3

X > 8024.3/25.1

So that, X > 319.69

Hence, atleast 320 random reads should be made to search the file