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Low write performance of NAND flash
The other severe handicap of NAND flash is its slanted read-write performance ratio (see "NAND flash solid-state drives vs. disk," below). While enterprise SSDs are capable of delivering a sustained read performance greater than 40,000 I/Os per second, write performance typically lags by a factor of three or four. This discrepancy is caused by NAND flash's requirement to erase blocks before they can be written, adding substantial overhead. That's also the reason why NAND flash storage shows significantly higher write performance as long as erased cells are available, but declines by a factor of two to three thereafter.
|NAND flash solid-state drives vs. disk|
"Since NAND flash-based SSD products show great write performance the first 15 to 20 minutes, it's pertinent to compare their sustainable performance rather than their inflated burst performance," cautioned Woody Hutsell, president at Texas Memory Systems.
The STEC Zeus IOPS solid-state drive with its maximum 52,000 sustainable read IOPS and 17,000 write IOPS, according to the company, currently dominates in the enterprise storage space and has established a baseline that other SSD offerings are compared to. "With the latest STEC drives, write performance is enterprise ready, but clearly not on par with read performance," said Kyle Fitze, marketing director, HP StorageWorks Storage Platforms Division. Unfortunately, no independent third-party tests for enterprise-grade SSD products are available at this point and, as a result, performance numbers cited by vendors should be taken with a grain of salt.
To overcome the read-write performance gap, most vendors are deploying a small DRAM cache that acts as a write buffer; that is, data is written to the cache first and then to NAND flash. "A DRAM write buffer doesn't quite get you to read performance, but it gets you closer," noted Clod Barrera, chief technical strategist for IBM System Storage. Even though DRAM helps close the gap, it adds the complexity of having to back up the volatile data in cache in case of a power failure and consumes valuable real estate. "As we move to 1.8-inch drives and custom form factors, you want to enable the highest density, and DRAM clearly is prohibitive," said Thad Omura, vice president of marketing at SandForce Inc., a developer of SSD processors.
Because of these shortcomings and the inability to completely make up for the write handicap of NAND flash, newer and more innovative solid-state drive developments shun the DRAM write buffer approach. Pliant Technology Inc. claims to achieve more than 100,000 IOPS for both reads and writes. Commercially available, the Texas Memory Systems RamSan-620 is capable of sustaining 250,000 IOPS for both reads and writes, according to the firm's Hutsell. Similar to the RamSan-620 and Pliant Technology's Enterprise Flash Drives (EFDs), SandForce's SF-1000 family of SSD processors, which interface to both MLC and SLC drives, forego a DRAM write buffer. All three vendors emphasize the role of parallelization as key to overcoming the write performance gap and an overall increase in the number of supported IOPS.
"It's our custom parallel-processor architected ASIC that enables us to perform many of the housekeeping tasks, such as pre-erasing of unused blocks, concurrently, and it enables us to get write performance in line with read performance," explained Greg Goelz, vice president of marketing at Pliant Technology.
This was first published in September 2009