Are Solid-State Hard Drives (SSD) in Your Future?

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In our computer dependent world, we are always on the lookout for the next “big thing”. Often times it has been a new microprocessor from Intel or AMD. However, in recent years, the microprocessor is often not the limiting factor with computer performance.

While hard-drive memory capacities have increased significantly in recent years, the access speed has remained relatively constant. It’s an issue/problem fundamental to the technology. The hard-drive is mechanical, and efforts to improve speed have been receiving diminishing returns.

A technology that is competing with the conventional hard-drive (HDD) is the solid-state drive (SSD). An SSD is not mechanical, it is based on “flash memory”, the same computer chip technology used to store pictures with your digital camera. An SSD drive was a novelty only 3 years ago, but no longer.

An SSD can outperform conventional mechanical hard drives because it is 4X smaller and lighter, is up to 50X faster, is more reliable because there are no moving parts, produces less heat, and uses less power. SSD drive memory capacity has improved and 250 gigabyte SSD drives are now available. Currently, the drawback is price.

SSD Drives are still more expensive than conventional HDD. An SSD Drive now costs about $2 per gigabyte while a HDD drive costs less than $1 per gigabyte. The price of an SSD continues to improve (SSD drives once sold for more than $25 per gigabyte), and with manufacturing volumes anticipated, the price difference should be further reduced.

Flash memory is capable of a finite number of rewrites to each memory cell (as are conventional hard drives). Significant improvements in the technology have been made in recent years. In addition, Intel developed “load leveling”. This technique ensures that all of the memory cells on the SSD receive a similar workload. Most SSD manufacturers now utilize comparable techniques. An SSD should last 10 years or more for the average user.

The primary reason for the rosy future of SSD is access speed. A fast conventional HDD has access times equal to about 5 milliseconds. It sounds fast, but when the microprocessor is capable of millions of instructions per second (MIPS), 5 milliseconds is a bottleneck. SSD can have as little as 100 microsecond access time (50X faster).

An important issue when trying to utilize this SSD speed capability is the potential bottleneck caused by the interface. There are 3 common interfaces used today with SSD drives.

The SATA interface is currently the most common interface used for conventional HDD, but a SATA interface has been limited in total throughput, both send and receive, to about 3 Gbps. This can be too slow for SSD, causing performance disruptions. Some SSD drives are capable of over 5 Gbps throughput.

Seagate Technology, in conjunction with AMD, recently announced the Serial ATA 6-Gbps storage interface, also called SATA Revision 3.0, a next-generation technology that is capable of twice the speed of the fastest SATA interface available today. This technology was demonstrated for conventional hard drives, but has obvious application to the SSD market.

SAS (Serial Attached SCSI) is another interface alternative. SAS is a point-to-point technology with at least four channels. Each channel is capable of throughput of 3 Gbps in each direction (a total of 6 Gbps per channel).

A third alternative is to implement the SSD with a PCI Express interface. A PCI Express interface has unidirectional data paths, one send and one receive, each at 2.5 Gbps for a throughput of 5 Gbps.

It is possible to maximize the performance advantage of the SSD technology with careful selection of the appropriate interface.


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