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Exercises – Disks 3 / 4 / 2012 1. Exercise: computation of access time for RAID 0 disk architectures. A storage server consists of 10 disks with RAID 0 organization. The disks have the following characteristics: an average seek time of 5ms, 7200 RPM rotation speed, average transfer rate of 8 MB/sec, 1KB of block size (stripe unit) and a stripe width of 4 disks. Compute the minimum time to access all the data of a file of 400 KB, assuming that all the disks have constant rotation speed and are synchronized. Controller time is set to zero. Solution: Mean access time for 1 block = seek + latency (1/2 round) + transfer (1block) = 5 + (60 x 1000/7200)/2 + 1000/(8 x 1024) = 5 + 4.17 + 0.12 = 9.29 ms The access time to read al l the file with a single disk: 400 u 9.29/1000 = 3.716 sec Finally we can compute the minimum read time for the file, taking into account that there are 4 blocks each read, one per each disk in the stripe width: 3.716/4 = 0.929 sec 2. Exercise: arriving requests to a RAID 1. Let S be the mean service time for one I/O operation of a RAID 1 storage organization consisting of 2 disks, O the rate of arriving requests (per second) and Or / Ow = 3 the number of reads with respect to the writes. Considering that the two disks are mirrored, compute the total I/O rate for the RAID 1, then the I/O rate per each disk. Solution: Arriving I/O rate at the array: O=Or + Ow = (1 + 1/3)Or = 4/3 Or O=Or + 2˜Ow = Or (1 + 2/3) = 5/3 Or is the I/O rate on both the two disks RAID1 O=Or/2 + Ow = 5/6 Or is the read I/O rate per disk 3. Exercise: Mean Time to Data Loss of a RAID 0. A RAID 0 array consists of 8 disks, the MTTF for each disk is 1000 days. The MTTR is 10 days. Which is the array’s MTTDL? Solution: MTTDL in this case is not affected by MTTR! MTTDL = MTTF…