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Throughput bottlenecks

In looking at storage, capacity is not the only issue. How fast you can get data from storage to a destination is also an issue. This path from storage to destination often travels several roads and each has its own limitations.

The Wikipedia SATA page has a nice comparison chart for hard drive buses. Curent drives can transfer data at a maximum of about 120 MB/s which is covered by the standard SATA or PATA buses. The Firewire and USB ports often found on current motherboards can handle data at about half the hard drive speed or at about 50 to 60 MB/s. The device bandwidth page notes that the old 300 baud modems that could handle 30 B/s were at human reading speed. That means current buses operate at about a million times human reading speed.

A storage device might also need to traverse a local area network. Most common is Fast Ethernet which can handle about 10 MB/s which slows down to about a third that in practical situations. If Wireless is involved, the speed drops to below a quarter MB/s with the latest and greatest standards maybe hitting 10 MB/s or so.

For comparison, the standard bus in the computer, the PCI bus, can handle 130 MB/s and the new version, PCI express, can handle 250 MB/s for each of up to 16 channels. The memory buses tend to run from about 1000 MB/s (5 year old PC133) to 6,400 MB/s (PC2-6400) in modern mid level PC’s. Digital audio runs about a third of a MB/s and the latest HDMI video with 1080p runs about 1,275 MB/s.

But, back to loading up a storage device. The two buses usually used are USB and fast Ethernet. USB looks to be about 5 times as fast. For time, let’s consider 100 GB and the slower ‘fast Ethernet’ in practical terms. Netgear gets actual speeds of about 20 MB/s in tests for its NAS devices, which are rated as speedier than most, using gigabit Ethernet. Fast Ethernet runs at a tenth of that speed so consider about 3 MB/s as a good rate for calculation. 100 GB is about 1E5 M bytes divided by 3 MB/s yields 5E4 seconds, which is about a full day.

The bottom line

If you are looking at loading up an external storage device with the typical wires between boxes approach, plan on it taking a while. A 500 GB external drive can take nearly a day to fill over a LAN or USB link going full tilt. Even drive to drive inside the box you are still talking about tens of minutes. Modern storage devices hold a lot of stuff!

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