One can uncover the chunk size with the following code. The basic idea is to perform reads from N patterns chosen at random, but always at controlled offsets, c and c - 1, within the pattern.

Figure 6.27 Results from running the chunk size algorithm of Shear on a mock storage system.

If you run this code and plot the measured time as a function of c, then you will see that the measured time is lowest when the requestA and requestB reads fall on two different disks. Thus, the values of c with low times correspond to the chunk boundaries between disks of the RAID.
a. Figure 6.27 shows the results of running the chunk size algorithm on an unknown RAID system.
€¢ What is the chunk size of this storage system?
€¢ What do the measured times of 0.75 and 1.5 seconds correspond to in this storage system?
b. Draw the graph that would result from running this Shear code on a storage system with the following characteristics:
Number of requests: N = 1000
€¢ Time for a random read on disk: 5 ms
€¢ RAID level: RAID 0
€¢ Number of disks: 8
€¢ Chunk size: 12 KB

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for (c = 0; c < pattern size; c += BLOCKSIZE) { for (1-0; i < N; i++) { requestA[i] request[i] (c-1 = random()*pattern size + c; = random()*pattern size + pattern size; 1.5 pa--キ キキ+. 1.0 0.5 トー· 0.0 16 32 48 64 Boundary onset assumed (KB) begin time -gettime) issue all requestA[N d requestB[N] to raw device in parallel; wait for requestA[N] and requestB[N] to complete; interval_time gettime() begin_time; printf("ChunkSize: %d Time: %d\n", c, interval time); -