Although there are a large number of metrics that can be used to define quality of service (QoS) in a SAN environment (in part because QoS is a slippery concept) the Clipper Group identifies three main ones: Performance, availability and cost.
Performance measures how quickly the SAN accesses storage. Typically this is measured either by bandwidth or throughput. Bandwidth (the total capacity of the channel per unit of time) is more appropriate when data moves in large chunks, such as video files. Throughput (I/Os per second) is a better measure when looking at transactions and other areas where moves in smaller blocks. In most applications throughput is more directly related to access time.
Availability measures how often the data is unavailable and is usually expressed as the percentage of time the data is available to users. Availability includes not only how quickly the SAN can recover from a disaster, but, more importantly, how quickly it can recover from everyday glitches, such as a corrupted data block as well as the frequency of scheduled outages for maintenance, upgrades and such.
Cost is most easily measured by cost of acquisition but most accurately reflected by the total cost of ownership (TCO). TCO is especially important because it includes the ongoing operating costs, which are often significantly higher than the purchase cost over the life of the SAN.
Clipper Group discusses QOS in its paper "Tiered Storage Classes Save Money -- Getting The Most Out Of Your Storage Infrastructure" available at: www.clipper.com/bulletins/2002/Tiered_Storage_final.pdf.
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.