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July 13, 2007
• Vol.29 Issue 28
Page(s) 24 in print issue
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Solid State Disks: Costs, Sizes & Other Issues
Are Traditional Disks Obsolete?
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Could traditional hard drives be about to suffer the same fate as the floppy diskette? Floppy diskettes were once a ubiquitous method of storing data; nowadays, it’s difficult to find a computer or laptop with a floppy drive.
Today, solid-state disks, or SSDs, are rapidly gaining ground as a viable storage technology and an alternative to traditional hard disk technology. Even though SSDs today are more expensive and have smaller capacities than their larger hard disk brethren, one thing is certain: They will get better. So, is SSD technology worth keeping your eye on as a potential storage technology with real advantages? Or, are hard disks’ current capacity and cost advantages too large to overcome?
 No Moving Parts = Advantage SSDs?
SSD proponents point out that the Achilles’ heel of modern HDDs (hard disk drives) is the fact that they still use mechanical parts—rotating platters and read/write heads—to get the job done. So, access times for an HDD are determined by physical limitations, such as the time it takes for a platter to spin to the desired location (rotational latency) and the time it takes for a read/write head to move to the desired location (seek latency). The sum of these two times equals the access time for an HDD.
According to findings from Solid Data Systems (www.soliddata.com), today’s fastest HDDs have access times totaling 4 milliseconds. Seems like quite a short time, but when you consider the fact that RAID systems are the storage cornerstone of modern business applications that rely on transaction volume to perform effectively, any bottleneck results in real performance hits (and real dollars lost).
SSDs, by their very nature, contain no moving parts. So, says Wade Tuma, CEO of Solid Data Systems, “DRAM drives are low latency, which dramatically improves application throughput per server.” SSDs feature access times in the 10-microsecond range, roughly 500 times faster than HDD access times, according to findings documented in a whitepaper by Solid State Systems. These faster access times pay dividends when used with high-transaction systems, such as corporate databases, because faster access times mean database servers can process more transactions per unit time.
But, adds Tuma, it’s important to note that these performance figures apply to SDRAM-based drives and not flash drives, which are limited in write performance and may not be faster than rotating disks on random writes.
In fact, says Solid Data Systems’ Tuma, when measured in terms of price per transaction, SSDs are already cheaper than traditional HDDs. So, says Tuma, measuring performance in terms of cost per byte is unrealistic for SSDs because this metric does not take into account the cost advantages that SSDs bring to the table when used in high-transaction rate systems.
SSD Shortcomings
According to Cheen Liao, president of Synology (www.synology.com), SSDs have relatively slower write speeds due to the fact that current file systems are not designed to take advantage of SSDs. So, adds Liao, “file systems are needed to adapt to SSDs.” Today, says Liao, capacity and write speed are the main concerns surrounding SSDs.
Western Digital (www.westerndigital.com), a manufacturer of HDDs and other storage devices, is not convinced that SSDs are a tidal wave about to sweep HDDs away from the storage landscape.
“We believe,” says Steve Shattuck with Western Digital, “that much of the chatter about solid-state disks [SSDs] has been presented without a solid, lab-tested foundation.”
For example, he adds, SSD proponents have pointed out that SSDs are lighter, quieter, sturdier, and more energy-efficient than HDDs. But, Shattuck points out, Western Digital’s comparisons have indicated that SSDs are lighter than HDDs by only 15 grams. And, while Western Digital concedes SSDs are indeed quieter, Shattuck points out that HDDs in notebooks, for example, are nearly inaudible and produce about 22dBA of noise.
In terms of energy efficiency, adds Shattuck, SSDs and HDDs both do not use significant amounts of energy—such as notebook battery power—to make a significant difference. And in terms of reliability, he says, NAND flash-based SSD technology can only achieve about 100,000 read/ write cycles, a stark contrast to the 10 quadrillion cycles HDDs can achieve over their lifetimes.
“It will be interesting,” says Shattuck, “to see how SSDs perform over time in computing applications.”
The Future
Today, SSDs are more expensive and have less capacity than traditional HDDs. But, is that about to change, and if so, how soon?
According to Shattuck, “Western Digital does not believe that the significant hard drive capacity-per-cost advantage over SSDs will change dramatically over the next few years, especially with mainstream hard drive form factors, including 3.5-inch and 2.5-inch.” Also, he adds, hard drive densities are growing at solid rates (at least 40% per year), and high-capacity storage is in greater demand than ever before with new applications such as video, music, and photography.
According to Solid Data Systems’ Tuma, solid-state disks and hard drives will both continue to get cheaper, so pricing is not likely to even out. But, he adds, price per byte should not be the metric used to compare SSDs to HDDs because SSDs are already cheaper when measured in terms of price per transaction.
“SDRAM-based disks,” says Tuma, “are the best way to increase transaction performance and to prevent server overload during peak periods. So, they are a lifesaver in high-transaction-rate apps.”
In the future, he says, SSDs and disks will co-exist much as disks still co-exist with tape. But, Tuma predicts, SSDs will “. . . sit at the peak of the storage hierarchy.”
The Bottom Line
One thing’s for sure: SSDs are here to stay and will become a viable and valid alternative for storage. But, one technology will not completely supplant the other in the data center; instead, both will co-exist as administrators deploy SSDs under certain circumstances and stick with HDDs under others. Data center administrators should review their needs and deploy SSDs where they bring the most value. In some cases, it may be wise to wait for SSD technology to catch up in terms of both capacity and cost. 
by Sixto Ortiz Jr.
Solid-State Disks vs. Hard Disk Drives | SSDs (solid-state disks) | Advantages
• No moving parts (potentially faster and less chances for mechanical failure)
• Lighter (but not by much)
• Less noisy
• SDRAM-based SSDs better for high-volume transactions | Disadvantages
• Less capacity than HDDs
• More expensive (in terms of cost per byte)
• Less read/write cycles over lifetime (for NAND flash storage)
• Relatively slower write speeds than HDDs
• Higher manufacturing costs |
| HDDs (hard disk drives) | Advantages
• More established in marketplace
• Work well with modern file systems
• Cheaper (in terms of cost per byte)
• Larger capacities that continue to grow
| Disadvantages
• Moving parts that limit access times and are prone to failure
• Limiting factor for high-transaction-volume applications
• Less expensive manufacturing keeps cost structure lower | |
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