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Solid-state disks solve special problems

By Rick Cook

The fastest mass storage devices you can buy today are solid-state disks (SSDs), which are many times faster than any kind of hard disk. For example, if a conventional hard disk has a total access time of 10-12 ms (including command and data transfer) in a particular system, an SSD in the same application would have a total access time of around 0.25 ms.

The drawback to SSDs is cost. On a cost per megabyte basis they are at least ten times as expensive as an equivalent hard disk. A solid-state disk is a collection of DRAM chips organized to appear as a SCSI hard disk to the computer. Because DRAM is volatile, the disks include some form of uninterruptible power supply and often a conventional hard disk to store the data in the event of a power loss.

They are available from a number of vendors, including Compaq and Quantum. While the prices of memory chips --and hence SSDs -- have dropped over the years, the price of hard disks has dropped as fast or faster. SSDs are more affordable than they were two or three years ago, but they are still expensive.

In spite of their high cost, SSDs have an important place in some storage architectures. They are outstanding at speeding up random writes, especially in the fairly small block sizes encountered in applications such as transaction processing. They can also boost database performance if the critical indexes are stored in them.

SSDs speed up random reads as well, but caching and the appropriate RAID level disk array can speed up reads almost as much as an SSD at a much lower cost.

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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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