Solid-State Disks: Moving from Luxury to Necessity

Present Day

Today we can buy a server with one or more 3GHz CPUs, and a number of disk drives that still get about a 5ms access time. Do you know what the result of this is? A CPU that idles along at under 20% utilization while I/O beats the disks to death. Yes, the disk drives are effectively holding that back that blazing fast CPU from achieving anywhere near its potential. Imagine dictating a novel to someone "writing" on stone with a hammer and chisel. Get the picture?

So what's the solution? It could very well be solid-state disks. Not for all applications of course, but there are quite a few that really need the performance advantages- and these applications are much more common than one might expect. Database logs and indexes are huge I/O bottlenecks, as are mail server queues. And there are many other everyday applications that are impacted by I/O performance to rotating disks.

So what about the cost? Sure, memory prices have fallen tremendously, but is it enough to make the solid-state disks affordable? In a word, yes. Prices have come down so far that solid-state disks can often be the fastest and cheapest way of solving an I/O problem. Is the cost per megabyte as low as a traditional disk? No. That's because of economics of quantity. There are thousands of rotating disks shipped for each dozen solid-state disks. That won't always be the case, but it is for the time being. While the cost of a solid-state disk is roughly 500 times that of a rotating disk, its performance is over 1000 times better. A disk's access time of even 5 milliseconds is an eternity compared to the access time of a solid-state disk, which is measured in microseconds³. That performance can unlock the horsepower of those fast CPUs you've just purchased.

We haven't just experienced changes in technology- we're also seeing paradigm shifts in computing in general. Traditionally the server has been the most expensive device in the machine room. That's not necessarily true anymore- the cost structures have changed. It may be unfamiliar to think of data storage costing more than the server, but now it's a fact of life. Don't think of a solid-state disk as being an unreasonable purchase because it's "more expensive than rotating disks." Think of it as a specialized acceleration device that's needed to fully realize the computer's performance. With new servers and storage (and their pricing), you're still getting much more power for the dollar than you used to.

Is it feasible to replace all of the rotating disks in an IT environment with solid-state disks? Certainly not, at least not today. Fortunately, there currently isn't a need to do so. Typically, only about 5% of the data in a given application is a "bottleneck." It's the data in those small key files that should be moved off of rotating storage. Adding solid-state disks to an IT environment has benefits beyond just speeding up accesses to those key files previously mentioned. When the "hot" files receiving most of the I/O are moved off of the traditional storage arrays, the cache in those arrays becomes much more efficient, since the "hot" data doesn't keep pushing other useful data out of it.

If that's not enough, think of the alternative ways of achieving more performance:

• Adding systems - The Gartner Group has shown that only 17% of an IT budget is hardware costs⁴. What adds up faster is the cost of additional software licenses, hardware maintenance, system administration, consumables (backup tapes, etc.), facilities (power, cooling, floor space) and so on.

• Adding more disk spindles - While this has been a tried-and-true method for enhancing performance, it has limiting returns. The more disks that are added to a stripe, the lower the MTBF goes⁵. Write performance can suffer if using a RAID level that has redundancy built in⁶. Finally, in a random I/O environment, no number of added disks will help- assuming the data's not in cache, accessing the data means that a disk still must spin, and a head must still seek.

• Adding memory to a server - This can help in some cases, but it also involves certain risks. Should the server go down for any reason, the data held in system memory is lost. Depending on the business, the effect can range from "inconvenience" all the way to "corporate catastrophe."

• Modifying the application - It may be possible to enhance the application to run more efficiently. However, these changes can take a long time and potentially introduce bugs into an otherwise working program. Even then the performance gains are only incremental at best. Besides, good software developers and tools can often cost far more than a solid-state disk.

SUMMARY

It can be seen that while solid-state disks were until recently considered "unobtanium," this simply isn't the case anymore. No longer are they to be considered a luxury to be used only by niche applications. Changes in economics and the computing industry have brought the price range into something manageable by most IT budgets, and performance demands now make the solid-state disk more of a necessity than a luxury.

³ A millisecond is 1000 microseconds.
⁴ Hardware costs are now probably an even lower percentage than Gartner's calculation of 17% due to prices having dropped so far in the past two years.
⁵ As the number of disks in a stripe goes up, the Mean Time Between Failures (MTBF) goes down exponentially. Since a solid-state disk doesn't have any moving parts, there's nothing to break.
⁶ Writing to a RAID stripe with redundancy (data parity) can add a tremendous amount of overhead. This is particularly so when writing less than an entire stripe as data must be read and parity recalculated.

Kelly Cash is the Technical Evangelist for BiTMICRO Networks, a leading supplier of flash-based solid-state disk storage solutions. He has been in the systems performance arena for 15 years, specializing in server and storage optimization. During his career he's been an engineer focusing on performance for such companies as Sun Microsystems, Data General, and AIM Technology. He's also held positions in the IT space, such as the Chief UNIX IT Technologist for Cadence Design Systems.

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