cache hit rate

The performance of a file system depends critically on the cache hit rate (fraction of the blocks found in the cache). If it takes 1 msec to satisfy a request from the cache, but 40 msec to satisfy a request if a disk read is needed, give a formula for the mean time required to satisfy a request if the hit rate is h. Plot this function for h:(0,1)

The mean time is calculated as the probability of a hit times the time required to proccess a hit plus the probability of a miss times the time required to proccess a miss or:

1 msec * h + (1 - h) * 40 msec