Solid-state disk lackluster for laptops, PCs

Laptops, desktops won't see a cost/benefit advantage in SSD for about two years

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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.

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