In either case, an SSD is a substantial amount of RAM, usually battery-backed DRAM, which is organized as if it were a very fast disk and attached to support hot spots of heavily used data. SSD makers such as Texas Memory Systems (
Adding solid state storage to a SAN is an expensive solution to a particular and rather narrow set of problems. If you have the right kind of performance issues, an SSD can do the job when nothing else will work. But other solutions are usually cheaper and are generally the first choice if they will do the job. For example, spreading the storage containing the application over several Fibre Channel loops will usually produce a significant performance enhancement at a lower cost.
The place to start considering an SSD to improve SAN performance is by carefully examining your SAN's performance using available diagnostic tools such as IOSTAT and other SAN management tools. A careful analysis of the reports will show if your problem can be solved by adding some faster memory. The next step is to apply alternatives such as tuning to try to get the needed improvements.
Then consider the size of SSD it will take to get the desired improvement, which can usually be determined from the storage reports. Although companies like Texas Memory Systems (http://www.superssd.com) offer SSDs holding up to a terabyte of memory, the higher price of solid state memory means that most SSDs will be much smaller, often not more than a gigabyte or so. If the SAN is bottlenecked by very specific kinds of files at specific points, and if both of those remain fairly constant drags on performance, an appropriately sized SSD configured to handle the right data can produce significant improvement in throughput.
Texas Memory has a number of white papers and analysts' reports on SSDs on SANs on its web site.
For more information:Tip: Solid-state disk gives SANs performance
Tip: SSD or memory for database files?
Tip: Should high performance storage mean high cost storage?
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.
This was first published in January 2004