CARDSPEED - Card Readers and Memory Cards

Special Tests for CompactFlash Cards

The resulting data rate mainly depends on the following conditions:

• Aligned versus unaligned read/write: In many cases, the resulting rate for unaligned read/write depends on the actual unalignment. This means that in general, there is one "best" aligned data rate, but different resulting data rates depending on the offset from the best alignment.
• Contiguous versus non-contiguous read/write: Non-contiguous read/write requires more context switches for whatever reasons, and therefore can significantly slow down the transfer rate.
• Small versus large data chunks: When testing on a Windows PC, you can't be sure how much impact operating system and mainboard have. However, different cards still show very different behaviour depending on the size of the data for each read/write call.

It is more or less impossible to test a certain card under all conditions. The selected test procedure includes 128 different test runs from the following combinations: Read/write, sequential/skipping, aligned/unaligned, small/large total size with eight different transfer sizes. The total size for small read/write is 1MB with transfer sizes from 1kB to 128kB, resulting in 1024×1kB to 8×128kB units. The total size for large read/write is 64MB with transfer sizes from 256kB to 32MB, resulting in 256×256kB to 2×32MB units.

The test results are combined into two graphs, one for all read tests, and one for all write tests. The read graphs for different cards look very similar, but the write graphs differ a lot.

Below is an example for a write graph...

Above: Sample graph from a write test run.

Colors: As usual, the graphs show read in red and write in white. These are the results for aligned sequential read/write. The other three colors are the same for both graphs: Yellow for unaligned sequential, blue for aligned skipping, brown for unaligned skipping.
Left/Right: The left half with thin lines shows the test results from a total of 1MB, the right half with thick lines shows the test results from a total of 64MB.
Values: The X-Axis shows the data size that was used for the read/write calls to the operating system.
Results: The Y-Axis shows the measured transfer rate, as usual in binary calculation, where 1MB/s = 2²⁰ Bytes per second. The scale is chosen according to the speed category of the card: 20MB/s for PIO-6 cards, 35MB/s for slow UDMA cards, 55MB/s for the "first generation" UDMA cards, and 100MB/s for the "second generation" UDMA cards.

Below: Sample graph from a read test run.

For the moment, I don't want to draw any specific conclusions. The test runs are in progress, graphs will be created and publised step by step.