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Solid State Drive Applications in Storage and Embedded Systems

Sam Siewert is CTO of Atrato and has worked as a systems and software architect in the aerospace, telecommunications, digital cable, and storage industries. Dane Nelson is a field applications engineer at Intel.

Flash memory, especially NAND flash memory, has been steadily encroaching into new markets as the density has increased and the cost per gigabyte (GB) has decreased. First it was digital cameras, then cell phones, portable music players, removable digital storage, and now we are seeing the emergence of NAND based solid state drives (SSDs) in the consumer PC market. Some industry analysts predict that SSDs could be the single largest NAND market segment (in billions of GB shipped) by 2010.

The combined performance, reliability, and power of SSDs compared to traditional hard disk drives (HDD), explains the attraction of SSDs in the consumer marketplace.

Intel has launched a line of high-performance NAND based solid state drives; the Intel X25-M and X18-M Mainstream SATA Solid-State Drives utilizing MLC NAND, and Intel X25-E SATA Solid-State Drive utilizing SLC NAND. The higher performance density and lower price point make them a practical choice for the storage and embedded markets.

Solid State Drives versus Hard Disk Drives

Solid State Drives have no moving parts, unlike HDDs. The rotating media and servo-actuated read/write heads used to access HDD data are subject to mechanical failure and introduce seek and rotate latency. Capacity growth due to areal density advancement and low cost per gigabyte stored have been the main advantages of HDDs, but fast random access has always been a significant limitation.

The main difference between an HDD and SSD is the physical media in which the data is stored. A HDD has platters that encode digital data with magnetically charged media. These magnetic platters spin at a high rate of speed (5,400, 7,200, or 15,000 revolutions per minute, or RPM) so that a servo-controlled read/write head can be positioned over the cylinders/tracks of sector data for data access. In an SSD the digital data is stored directly in silicon NAND flash memory devices. An SSD has no mechanical moving parts, which improves the durability in resisting physical shock or mechanical failure and increases performance density. An SSD then uses a controller to emulate a mechanical hard disk drive, making it a direct replacement for mechanical hard disk drives but with much faster data access due to the lack of the servo positioning latency in HDDs.

In addition to the memory, a solid state drive contains; an interface connector and controller, memory subsystem and controller, and a circuit board where all the electronics are housed.

The main performance difference between HDDs and SSDs has to do with the limitation of the HDDs caused by the spinning mechanical platters. Two performance metrics that improve greatly are the random reads and writes per second, and the time it takes to enter and resume from a low power state. Random read and write performance is measured in inputs/outputs per second (IOPs). This is simply the number of reads or writes that can be completed in one second. A typical high performance 15-K RPM SAS hard drive can usually complete about 300 IOPs of random 4-kilobyte (KB) data. By comparison, the Intel X25-E SATA Solid-State Drive is able to process over 35,000 random 4-KB read IOPs, a difference of 117 times. The reason for this is that logical data locations on an HDD are directly mapped to ordered physical locations on the spinning physical disks. To access (read or write) that data, the disk must spin around to the correct location and the read/write head must move to the correct radius to access the data. Therefore, random data accesses require multiple ordered physical movements incurring significant mechanical access latency and significantly limiting performance. In a NAND SSD the data is stored in a virtual memory map on the NAND flash. Accessing any part of that is as simple as changing the address and executing the next read or write. Since most workloads are random in nature, especially as industries move toward multi-core compute engines, multithreaded operating systems, and virtual machines, random disk performance will only increase in importance.

The other main performance difference is the ability to resume operation from a low power state quickly. The lower power state for a HDD is accomplished by parking the read/write head off to the side and stopping the spinning platter. When the next read or write is requested the platter needs to be spun back up and the head has to be moved back in place, which can take on the order of seconds. In an SSD however, when it is not processing read or write requests, it can put itself into a low power state (through Device Initiated Power Management, or DIPM) and recover within a few milliseconds to service the next request. Hard drives take closer to seconds to do this, so they do not take full advantage of DIPM.

With SSDs' higher performance, added reliability, and lower power, one may ask why they have not completely displaced HDDs. The main reason is cost, because SSDs cost many times more per gigabyte than mechanical hard drives today. There has been early adoption in markets that absolutely must have the performance, reliability, lower power, or resilience to shock and vibration, but mass consumer adoption will only happen when the cost of a comparably sized device approaches that of a Small Form Factor (SFF) hard disk drive.

The second barrier is capacity, because SSDs typically have much smaller capacity than mechanical hard drives. As NAND flash densities increase, however, the capacity of SSDs will be large enough for most consumer, enterprise, and embedded marketplace needs.

Third, the limited number of write cycles per storage cell is a barrier to applications that mostly ingest data (50-percent writes, 50-percent reads) for later access. Flash memory, as it is erased and rewritten, will lose the capability to hold a charge after many program/erase cycles. This makes flash memory a consumable resource. However, with increased density, along with more comprehensive write wear-leveling algorithms, the longevity of solid state drives has improved.

Just because SSDs have an inherent advantage over HDDs in random read/write performance and in resumption from low power states doesn't mean that SSDs are better than HDDs in every case. Hard disk drives are excellent for storing massive amounts of data such as movies, music, and large amounts of digital content in general, due to their very low cost per gigabyte and continued improvements in areal density (gigabits/square-inch). Areal density improvements in HDD media technology have in fact followed or exceeded Moore's Law (capacity density doubling every 18 months or less), but access to that data has not improved at the same pace. The servo and rotational latency for access to data in HDDs has in fact been nearly the same for decades if you look at year-over-year improvements.

Solid state drives, on the other hand, excel when the system requirements are more skewed toward performance, reliability, or power. The performance and power metrics were described above, and the reliability of NAND storage in SSDs has been shown to be many times more reliable than mechanical hard disk drives especially in harsh environments.

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