Caching and SCSI Throughput

There are several variables that determine SCSI throughput: how fast your Mac can move data over the SCSI bus, how fast your drive can move data over the SCSI bus, and caching (including both disk caching and the disk's internal data buffer).

For this test I have a couple external drives, an older Quantum 40 MB and a relatively new Quantum 2050 MB) configured with System 7.5.5, allowing a standard configuration on all tested computers. I use Speedometer 3.06, since it runs on even the oldest Macs (such as my SE).

Note that Speedometer uses a 1 MB file for testing the drive; a cache larger than 1 MB makes no appreciable difference. Ratings are relative to the Mac Classic. (More data will be added as I accumulate it.)

You will obtain slightly different results at different cache sizes. There is also a bad interaction between cache size and disk buffer for drives with larger buffers, as the figure below shows. This data was obtained on a Macintosh IIsi using different drives.

Older drives with small (usually 64 KB or less) buffers have consistent performance until the disk cache hits 1 MB, the size of Speedometer's test file. At that point, performance depends on the cache, not the drive mechanism. The Quantum 2050, with its larger buffer, drops 30% between a 128 KB and 512 KB cache.

But note the consistent performance with a 1 MB or larger cache. Whether the drive itself is slow or fast, performance jumps to about 7 on the IIsi. That's 3.5x faster than the Q40, nearly 3x faster for the Q80, and almost 2x faster for the Q2050.

Perhaps the most interesting discovery is that the disk cache has a strong negative impact on the RAM disk created by ramBunctious. Performance dips drastically at 256 KB and 512 KB, leveling off at around 7 with a 1 MB or larger cache. Although the RAM disk is over 2x faster than the cache, the large cache drops performance to match that of cached hard drives.

Remember that these findings are true only for the Speedometer benchmark, which may not emulate the way you access data in the real world.

Still, these tests demonstrate a large enough data cache can make a real improvement on data throughput and that RAM disks, when used, should be used with a relatively small disk cache. (Ideally the cache would ignore a RAM disk.)

Although Speedometer doesn't show it, the caching provided by Speed Doubler provides better disk caching than the one that ships with the Mac OS as part of the memory control panel.

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