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Web Caching by Duane Wessels

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Performance Bottlenecks

The performance of a proxy cache system depends on numerous factors. Any one of the system components can become a bottleneck. In some cases, it’s possible to use benchmarking tools to identify the bottlenecks. For example, you can benchmark two different hardware configurations and compare their results. Does a faster CPU improve response time? How much does your hit ratio improve if you double the disk space?

Disk Throughput

Disk I/O is often a bottleneck for web caches. Due to the nature of web traffic, web caches make very heavy use of their disk drives. If you’ve ever looked at the disk lights on a busy cache, you probably noticed the lights are almost constantly on.

The performance of disk systems varies greatly between products, and a number of factors are involved. First, the controller’s bus speed limits how quickly chunks of data move from memory to the disk controller. For SCSI, this limit is usually 20, 40, or 80MB per second. For IDE controllers, the limit is either 33, 66, or 100MB per second. These rates correspond to theoretical upper limits. It’s rare for a controller to sustain its maximum rate for any length of time, partly because individual disk drives have much lower limits. When multiple devices are connected to the bus, as is common with SCSI, then the bus bandwidth limit becomes more relevant.

Another, and more realistic, limit is how quickly an individual drive can transfer data to and from its magnetic media. This limit relates ...

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