At first glance, solid-state drives (SSDs) appear to be a no-brainer for makers of storage systems for enterprise servers and laptops. After all, SSDs promise higher read/write performance, higher reliability, and lower power consumption when compared with hard-disk drives. But in practice, SSD adoption has been held back not only by a higher cost per gigabyte, but also by real-world issues that prevent them from achieving their performance and reliability promises.
In this article, we will look at the key technical issues preventing SSDs from broad-based adoption by enterprise server and laptop makers, and outline the requirements for broad-based market success.
SSD TECHNOLOGY TODAY
Last year saw the proliferation of SSD product announcements in enterprise-class servers and laptops, beginning with 32- and 64-Gbyte devices. In servers, we’ve seen SSDs used in so-called “Tiered Storage” systems, in which the SSD acts as a higher-speed intermediary between system RAM and hard-disk-drive (HDD) storage. Fujitsu and other vendors have also begun to use SSDs in enterprise-class laptops, touting their ruggedness and higher read performance.
With SSDs now in the marketplace, two key trends have begun:
Declining costs: As NAND flash manufacturers have continued to advance technology and densities, the price per gigabyte from NAND flash vendors has dropped approximately three orders of magnitude over the last decade, starting from thousands of dollars in the 2000 timeframe to today’s commodity price levels of around a $1 per gigabyte (for MLC-based technologies). Continued price declines are expected for years to come.
Increasing performance: At the same time, advances in flash memory as well as techniques such as DRAM caching are driving input/output operations per second (IOPS) higher, with today’s fastest SSDs sporting tens of thousands of read IOPS.
SDD CHALLENGES
Despite these advances, most analysts predict a very slow ramp up toward broad-based adoption of SSDs in enterprise-class servers and laptops. One key reason is the relatively high cost per gigabyte for SSDs (compared with HDDs). Today’s SSDs mainly use SLC memory due to its higher life expectancy and reliability.
The cost of SLC memory is roughly four times higher than multilevel-cell (MLC) memory due to two factors. First, MLC memory stores two bits per cell and therefore provides twice the storage per square millimeter of silicon (the main cost of the memory). Second, the volume of MLC is roughly 90% of all NAND flash, further increasing the economies of scale in its production. Unfortunately, MLC flash memory isn’t yet deemed reliable or durable enough for widespread enterprise use.
Nevertheless, MLC flash is clearly the way forward due to its ability to rapidly reduce the cost per gigabyte. Still, several challenges must be overcome when using MLC flash in its current implementation:
Poor write endurance: NAND flash memory can only be written a certain number of times to each block (or cell). SLC memory generally sustains 100,000 program/erase (P/E) cycles, while MLC memory is generally ten times less at 10,000 cycles. Once a block (or cell) is written to its limit, the block starts to forget what is stored.
Today’s SSDs are different from HDDs when it comes to data storage. HDDs can take the data directly from the host and write it to the rotating media. In contrast, SSDs can’t write a single bit of information without first erasing and then rewriting very large blocks of data at one time (also referred to as P/E). In addition, to maximize the life of the flash memory, a technique to level the wear across all blocks equally forces the SSD controller to constantly move data around on the flash memory.
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