Concepts illustrated by this case study

■ RAID Systems

■ Mean Time to Failure (MTTF)

■ Performance and Reliability Trade-offs

In this case study, you will design an I/O subsystem, given a monetary budget. Your system will have a minimum required capacity and you will optimize for performance, reliability, or both. You are free to use as many disks and controllers as fit within your budget.

Here are your building blocks:

■ A 10,000 MIPS CPU costing $1000. Its MTTF is 1,000,000 hours.

■ A 1000 MB/sec I/O bus with room for 20 Ultra320 SCSI buses and controllers.

■ Ultra320 SCSI buses that can transfer 320 MB/sec and support up to 15 disks per bus (these are also called SCSI strings). The SCSI cable MTTF is 1,000,000 hours.

■ An Ultra320 SCSI controller that is capable of 50,000 IOPS, costs $250, and has an MTTF of 500,000 hours.

■ A $2000 enclosure supplying power and cooling to up to eight disks. The enclosure MTTF is 1,000,000 hours, the fan MTTF is 200,000 hours, and the power supply MTTF is 200,000 hours.

■ The SCSI disks described in Figure 6.3.

■ Replacing any failed component requires 24 hours. You may make the following assumptions about your workload:

■ The operating system requires 70,000 CPU instructions for each disk I/O.

■ The workload consists of many concurrent, random I/Os, with an average size of 16 KB.

All of your constructed systems must have the following properties:

■ You have a monetary budget of $28,000.

■ You must provide at least 1 TB of capacity.

Figure 6.3

Assume that you now have access to a disk that has twice the capacity, for the same price. If you continue to design only for reliability, how would you change the configuration of your storage system? Why?


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