July Reader Challenge Solutions

Eclectic Autos’ IS department needs to upgrade its hardware. The Spit and Polish division’s Windows NT 4.0 server needs upgrading. This server holds two databases: the database the spitters and polishers use to look up suppliers, part numbers, and prices, and the database in which they enter details about each job they perform. The computer is also the department's (Microsoft Exchange) email server.

The spitters and polishers are concerned about the database’s accessibility and integrity, and they’re especially worried about email problems. The IS codirectors, Larry and Moe, have a budget that isn't large enough to cover everything they want. Larry and Moe agree that their system must be reliable (i.e., fault tolerant), but they can’t agree on how to best achieve reliability. Larry prefers mirror sets, and Moe insists on stripe sets with parity.

One of Larry and Moe’s arguments follows. See how much misinformation you can identify.

Larry: Mirror sets are faster because they write the data set whenever they have a free nanosecond. Mirror sets need only two disks, whereas stripe sets with parity need at least four disks.

Mirror sets are often faster than stripe sets with parity, but not fast enough for you to discount stripe sets. Stripe sets with parity need at least three disks (maximum 32).

Moe: But stripe sets with parity are perfect for high-speed data writing because they write symmetrically. Our email server has periods of high-speed writing and not much reading, so stripe sets with parity are a perfect solution for our server.

Stripe sets with parity are preferable if you have more reads than writes. Performance can decrease as the percentage of writes grows. In this case, you need extra I/O operations to write the parity strip, and I/O is thus unbalanced. Network email situations with a lot of high-speed writing and not much reading rarely if ever exist.

Larry: If we use mirror sets we can use the disks in the storage room.

Moe: No we can't, because mirror sets require identical disks, and the disks in the storage room are not identical.

Larry: Wrong; the disks don't have to be identical.

The disks do not have to be identical. The partitions are mirrored, not the disks.

Moe: You're wrong. Besides, if we use stripe sets with parity, we can easily add disks to enlarge the stripe sets if we need more disk space.

You cannot add disks. You must destroy the stripe set and start over.

Larry: Let's compromise. We can install six disks, using four disks we have. Thus, we'll need to purchase two disks, but they don't need to be large, expensive disks The first four disks will be stripe sets with parity, and the last two disks will be a mirror set.

Moe: You can't boot from stripe sets with parity, so the system files need to be on the first two disks, which must be mirror sets.

The best solution is to start with booting mirror sets. Booting from stripe sets with parity when you have mirror sets available generates extra work. Setting up a boot with stripe sets and performing a recovery in this type of setup are difficult.

Larry: OK, but we need to put the swap file on the stripe set with parity because it has more available disk space.

The best solution is to put the swap file on the mirror of the mirror set.

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