Solid-State Disk Lackluster for Laptops, PCs
Most observers agree that solid-state disk (SSD) will eventually overtake magnetic disk drives as the storage medium of choice. SSD is lighter than traditional hard disk drives, is faster, is more durable and consumes less power. Still, SSD doesn't measure up to the hype, particularly when using it in a desktop or laptop PC.
"There are a host of problems with SSD," says Avi Cohen, head of research at Avian Securities LLC in Boston. "There's no reason to pay the extra $600 to $800 -- or a 40% to 80% premium -- for a solid-state drive."
Cohen is not alone in his assessment of consumer-grade SSD. Consumer-grade SSD generally uses multilevel cell (MLC) NAND flash memory, which has greater capacity and a lower-price point but suffers from slower I/O and as much as 10 times fewer read/writes over its life span. Corporate-grade SSD uses single-level cell (SLC) NAND memory and multiple channels to increase data throughput and wear-leveling software to ensure data is distributed evenly in the drive rather than wearing out one group of cells over another. And, while some consumer-grade SSD is just now beginning to incorporate the latter features to increase its performance, there will still be a cost/capacity disparity for years to come.
Other analysts agree, and even disk drive makers, including Fujitsu Ltd., do not see themselves producing SSD for at least another two years. That's how long it will take before the cost-vs.-benefit ratio makes sense for SSD to be a viable alternative to hard disk drives in laptops and PCs.
"I think you need to get to 128GB for around $200, and that's going to happen around 2010. Also, the industry needs to effectively communicate why consumers or enterprise users should pay more for less storage," says Joseph Unsworth, an analyst at Gartner Inc., referring to the fact that a 1TB hard disk drive today can cost under $200. A 1TB SSD would cost tens of thousands of dollars, he says.
Today, consumer-grade SSD costs from $2 to $3.45 per gigabyte, hard drives about $0.38 per gigabyte, according to Gartner and iSuppli Corp. Just two years ago, SSD cost $17.50 per gigabyte, so it's obvious that consumer NAND flash memory will soon be a true contender to hard disk drives -- it's just not there yet.
"As a percentage of the whole solid-state drive industry, it'll remain pretty light for now," says Joel Hagberg, Fujitsu's vice president of business development. "Pricing needs to get better."
Intel Corp.'s and Micron Technology Inc.'s upcoming SSDs will be based on 32Gbit chip technology. The companies are expected to be the first to break the $1.00 a gigabyte barrier with their upcoming consumer SSD products, which will cost about $0.99 cents a gigabyte, according to Jim Handy, an analyst at Objective Analysis.
How SSD Works
Two types of NAND flash memory are used to make SSD: SLC, storing one bit per cell and MLC, storing two or more bits per cell. Even without any software or firmware enhancements, SLC memory is inherently faster, is more reliable and has greater longevity than MLC, Avian Security's Cohen says. On the other hand, SLC is also more costly to produce and stores significantly less data than MLC.
All SSD natively excels at sequential and random reads -- such as watching videos or listening to music -- because as long as there's free space, the operations require no additional processing to retrieve data. This is why SSD is an excellent choice for handhelds; these devices are used mostly to access music or video, with few data writes required.
NAND is not efficient at random writes. In fact, most vendors tout burst speeds when offering read and write rates without showing sustained sequential figures in their marketing materials, according to Cohen, Unsworth and others. To make up for this shortcoming, vendors are trying to navigate slow read/write speeds not through the NAND flash itself, but through the controller electronics, memory buffers, multiple controller channels, interleaving NAND chips in parallel and flash management software, according to Unsworth.
For example, this month Micron unveiled its newest SSD line for notebooks, the C100 and C200 models, which have from 32GB to 128GB of capacity. Micron states that the drives offer sustained read speeds of up to 250MB/sec. and write speeds of up to 100MB/sec.
The faster sequential write speeds are achieved through two methods: a DRAM buffer, and by increasing the number of I/O channels. The use of firmware tricks the application into believing that the data is being written randomly to the drive, when it's actually being remapped and written sequentially, says Gregory Wong, president of Forward Insights, a consulting and market research firm focused on nonvolatile semiconductor memories.
SSDs are vastly more efficient than hard disk drives in random reads because there is no actuator head (similar to a record player's needle) that must be positioned over the data for retrieval. For example, mechanical positioning latency on a 7,200-rpm hard disk drive can be as much as 5 or 6 milliseconds. Page read times, or access times, for SDD are about 100 times faster than hard disk drives.
"Sequential performance is easy to improve with a DRAM buffer. But if you look at a user profile on a PC, most operations are random," Wong says.
The problem associated with random writes on SSD is that NAND requires an application to find an empty block to write to. If there is no empty block, the application must actually erase the data before it can write to the block, creating about a 2-millisecond delay, which adds up to significant overhead, Wong says.
Another fundamental problem with NAND flash memory is something called write amplification. Data is not written to flash memory in the same way it is written to a host system. Instead, data is laid down in .5MB to 1MB blocks, so if a host requests a 4KB block deletion on a flash drive, anything on the order of 20 to 40 times as much data is written to the NAND flash memory as is written to the host, according to Knut Grimsrud, Intel's director of storage architecture.
"The result is that when you want to write 4KB, for example, you end up having to erase a megabyte's worth of space and then you have to put the data back in it that you didn't want to write, and so often times you end up writing a lot more to the NAND than you wanted to," Grimsrud says, adding that the process creates significant overhead.
SSD for the Enterprise
It is costly to optimize SSD for high-transactional operations, which requires sophisticated firmware and software within the drive's controller. STEC Inc.'s enterprise-class SSDs that it sells to EMC Corp. tout 52,000 transactions per second or input/outputs per seconds (IOPS). But a consumer-grade equivalent SSD drive achieves only 300 to 600 transactions per second, Forward Insights' Wong says. "It's not that they can't do better, but they aren't," he says.
Intel claims it has been able to mostly overcome the write-amplification and deliver 30 times more write performance to the host, or a 1.1 write amplification ratio. The company also says its new drives offer up to 35,000 operations per second. Grimsrud would not disclose how Intel overcame the write amplification issue, saying it is currently a "trade secret."
Gartner's Unsworth says Intel's flash drives use 10 channel controllers that optimize performance through interleaving the NAND flash memory chips in parallel for greater efficiency. Meanwhile, Intel's Grimsrud says the company's soon-to-be-released line of SSDs offers write and read speeds comparable to traditional hard disk drives.
Grimsrud, who was part of the team that developed the new High-Performance SATA Solid-State Drive product line, says Intel's laptop and PC SSD drives have up to 250MB/sec. sustained sequential read rates and 70MB/sec. sustained sequential write rates. The serial ATA SSD's random read rate is 35,000 IOPS and it has a random write rate of 3,300 IOPS.
So, applications that require more reads but fewer writes are seeing tremendous performance advantages through the use of SSD over traditional hard disk drives. In fact, most experts agree that SSD is far superior to using high-end, 15,000-rpm hard drives in Fibre Channel-attached storage devices.
According to Avian Securities' Cohen, high-end flash drives outpace high-end Fibre Channel drives at a 20:1 price/performance ratio because businesses must use as many as 20 15,000rpm hard disk drives in order to attain the random read performance of a single SSD drive.
For the cost of each short-stroke spinning disk drive that customers replace, "you could put 20 SSDs in," Cohen says.
SSD and Laptop Battery Power
SSD is also touted to extend laptop battery life. However, most experts point to tests that show power savings through SSD equates to an additional five to 30 minutes on an average laptop -- the monitor and CPU eat vastly more power than a computer's drive. And some independent online publications have published results of tests showing some SSDs actually use more battery power than traditional hard disk drives.
Installing SSDs in laptops and PCs also requires Microsoft Corp. to update its operating system to take full advantage of NAND flash's attributes. The necessary changes were not included in the most recent service pack, and contacts within Microsoft have not heard of any progress coming in 2009, according to Cohen.
The reason SSD performance is not optimized is because the Windows operating system handles data in 4KB chunks. While SSD is also optimized to receive data in 4KB chunks, today's SSD drives are shoehorned into traditional hard disk drive bays, which receive data in 512-byte chunks, according to Forward Insights' Wong.
"It appears [Microsoft] is more focused on increasing PC touch-screen capabilities than NAND integration," Avian Securities' Cohen wrote in a recent analysis note.
Cohen believes the standards necessary to take full advantage of NAND and its capabilities within an SSD are just now beginning to flow through the various committees -- so full-featured products probably won't hit shelves till mid-2009.
Even vendors that sell SSD admit that when it comes to speed, there's little advantage at the consumer level (see "Performance showdown: Flash drives versus hard disk drives").
"Some SSDs out there are slower in writes than hard disk drives. It depends on how much you're writing at once. If it's 10MB, then it's probably on par with hard disk drives, but if you're talking 1GB, probably not," Gartner's Unsworth says.
Pat Wilkison, vice president of business development at flash memory maker STEC Inc., says the performance in SSD products varies greatly. STEC sells high-end flash memory to enterprise-class storage companies such as EMC, which uses the product in its high-end Symmetrix and midrange Clariion storage arrays.
"The class of product EMC needs is fundamentally different from what notebooks need," Wilkison says. "The reality is that performance varies depending on the applications running on it. Random write speeds are horrible. And guess what? As PC users, writes are important."
Western Digital Corp.'s fastest PC hard disk drive is the 3.5-in., 10,000rpm VelociRaptor, which has 300GB capacity. According to Computerworld's tests, the VelociRaptor racked up a 250.3MB/sec. burst speed, the highest we've ever recorded for a mechanical drive. Its average read/write rate was 105.6MB/sec. List price: $300.
Western Digital's fastest laptop drive is 2.5-in., 7,200rpm Scorpio Black, which has up to 320GB capacity. According our tests, the drive's average read rate is 63.8MB/sec. and its burst read rate is a screaming 238.8MB/sec. List price: $230.
In April, Computerworld tested two other top hard disk drives against the two top SSD drives at the time. The results showed little advantage for the SSDs.
The drives included the following:
SSD Durability, Reliability
Experts also agree that a new standard needs to be established for measuring SSD reliability so the actual life span of flash memory can be more clearly defined.
"There are over 80 companies making SSDs and the quality is all over the map," Gartner's Unsworth says.
Western Digital's SSD drives claim a MTBF (mean time between failure) of 1.4 million hours. Intel claims its new Extreme SSD drives have a MTBF of 1.2 million hours.
However, experts say MTBF is an inaccurate and subjective way to measure drive reliability, because different applications put different amounts of wear on drives. A better measurement to determine longevity may be how many write cycles a drive can handle, or how many times you can write and erase data.
Although SSD does have a durability advantage over hard disk because there are no moving parts to break, the life span of flash memory varies greatly, depending on a number of factors.
For one thing, it matters whether the SSD drive uses SLC or MLC memory. SLC generally endures up to 100,000 write cycles or writes per cell, while MLC can endure anywhere from 1,000 to 10,000 writes before it begins to fail, according to Fujitsu's Hagberg. For its part, Western Digital's laptop hard-disk drive boasts up to 600,000 write cycles.
But SLC also costs about twice as much as MLC to produce, Hagberg says.
Some MLC memory, however, can be faster than SLC memory based on write-amplification correction software, which can boost raw flash chip performance but degrade longevity. With software enhancements, MLC can exceed SLC performance, but at its core, it's still MLC memory, which means its life span is greatly reduced because cells store more data more often. In fact, generally speaking, the higher performance in an SSD drive, the longer life it will have because of better drive efficiency.
But the myth that SSD drives can attain the same longevity as hard disk drives is true only in enterprise-class devices that have wear-leveling software in the drive's controller. This evenly distributes data across the device to ensure that cells do not wear out prematurely.
Hagberg says his company does not plan to launch any SSD drive products over the next two years because the value proposition of the technology is not compelling enough and won't be until technology breakthroughs change SSD's performance and reliability, as long as two years from now.
Still, in less than 10 years NAND flash sales have grown from insignificant numbers to become a $16 billion market, according to Objective Analysis. "This product, the fastest-growing technology in the history of the semiconductor market, is poised to displace DRAM as the leading semiconductor memory," Handy stated in a recent report.
Gartner expects the low-cost SSD category to grow from 635,000 units in 2007 to over 33 million units in 2012. That represents a five-year compound growth rate of 117%, Unsworth says.
For today, however, other than high-end storage arrays and servers, the only other place SSD will see marked growth is in handheld devices -- because their main purpose is reading data. SSDs are also expected to do relatively well in ultraportable laptops and high-end laptops such as Apple Inc.'s MacBook Air, Avian Securities' Cohen says, and only because "it's cool," and not because of any performance advantage.