Raid 0 A Zero?

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There’s a good article about RAID 0 over at Anandtech.

The conclusion? ” . . . [T]here is no place, and no need for a RAID-0 array on a desktop computer.

Since this will no doubt be a very controversial conclusion in at least some circles, we thought we’d say a few things about it, too.

A Few Tiny Observations

We will note that these tests were run on machines using 1Gb of RAM, which reduces the swapping to-and-fro between memory and the hard drive a bit.

We’ll also note that a dual Raptor gets fairly close to the 150Mb/sec maximum of SATA 1 and could in some instances exceed it, especially in bursts. The 300Mb/sec provided by SATA 2 would provide enough additional headroom to prevent that.

Nonetheless, these factors would have minimal impact, and would not change the results any more than a tiny bit, nor change the conclusions.

Why?

Designed Not To Be Used

In the world of electronics, hard drives are agonizingly, excruciatingly, obscenely slow.

Current CPUs perform operations in terms of nanoseconds. Hard drives operate in terms of milliseconds.

A millisecond is a million times longer than a nanosecond.

The real difference between CPU time and hard drive time isn’t quite that great since it takes a CPU a cluster of nanoseconds to complete an operation, while it takes a hard drive a number of milliseconds, but a ballpark realistic factor in the differences in time is still something like 100,000:1.

What does being 100,000 times slower mean in real time?

Imagine you’re on an assembly line and could do something in a second, but you have to wait more than a whole day to get the thing that has to be done. That’s the equivalent of how long a CPU has to wait for data to come from the hard drive.

Neither you nor the CPU are going to get much done if that’s the supply line, now will you?

Would matters really get much better if the wait time was cut down to fourteen hours?

This is why hard drives are designed not to fetch data at the point when it’s needed. They are designed to anticipate what the CPU will need next, fetch it before it is needed, and have it sitting in a cache to be ready to be grabbed by the CPU electronically rather than mechanically.

Even that’s pretty slow, though magnitudes better than relying on some moving part.

The whole infrastructure of the PC is designed to get data ready to be fed into the CPU before it is needed so it can be accessed immediately, and the closer the data can be prepositioned to the CPU, the better. When you take an action the system can reasonably anticipate you’re going to do, things happen almost instantly. When you don’t, you wait.

That’s essentially why RAID doesn’t do much for desktops, but does help servers. A PC can anticipate much of what you are likely to do if you’re using an application or game. A server can’t anticipate which customer service record a store is likely to need next to complete an on-line transaction.

So a desktop can often bypass the hard drive bottleneck, while a server can’t. If you bypass the bottleneck, it matters little how much of a bottleneck it is. If you can’t, the bottleneck determines how fast (or slow) you are.

If you’re on an assembly line, and the people running the factory order the parts you need ahead of time, does it matter to you if the parts take a half-day rather than a day to get there? Will you work any faster if it takes a half-day rather than a whole one? No, all that matters to you is having the part when you need it.

On the other hand, if the factory has to order the part only after you find out you need it, it would make a big difference if you only had to wait a half-day rather than a whole one for each part.

Oversimplified? Of course, but this is the core truth of the matter.

An Urban Legend

Storage Review is considered by many to be the authoritative source for hard drive information. They’ve seen the Anandtech review, and they agree with it.

They’ve been saying the same thing again and again, but as they put it, “Many readers refused to believe.”

This is what happens when people learn a million facts, but not core concepts. That only makes them fact-filled fools (well, fools once they’ve rejected the controlling core concepts in favor of their “facts.”). Facts are not of equal value.

Yes, a RAID 0 array pumps out data faster than a single drive, but that’s not the important fact here. The important fact is that how fast a snail is doesn’t matter when he’s being bypassed.

It’s like a martial arts expert facing off against someone in and thinking you’re going to kick the tank’s butt. Sure, you can move faster than the guy in the tank, but that doesn’t matter. What matters is that you’re hardly going to hurt the tank, and while the guy inside may not move faster than you, his bullets or shells certainly do.

Ed

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