Determining the SD card speed class

First, let's try to figure out what kind of characteristic is this: the speed class of an SD memory card (Speed ​​Class).

Actual recording speeds may vary from manufacturer to manufacturer, however a consistent minimum recording speed is important for recording streaming content (video for example). The Secure Digital Association, to help consumers select the right memory cards for their performance, has set the Speed ​​Class standards.

  • Class 2 - performance is greater than or equal to 2 MB / s
  • Class 4 - performance greater than or equal to 4 MB / s
  • Class 6 - performance greater than or equal to 6 MB / s
  • Class 10 - performance is greater than or equal to 10 MB / s

When ordering memory cards from the Middle Kingdom, there are often doubts about the class's compliance with the declared one. The largest Chinese marketplace alibaba even advises checking the speed with the free h2testw utility, some use the Crystal Disk Mark utility.

Sometimes it happens that it does not reach the declared speed quite a bit and a person begins to be indignant that his Chinese have deceived and sold the card a class lower.

It seems to be simple, clear and obvious, but there is a nuance. The fact is that according to the specification, when evaluating the performance of MB = 1000 × 1000 Bytes.

It became interesting whether these utilities take this evaluation feature into account.

Crystal Disk Mark is an open source program. A quick look at the source gave the result:

#define DISK_TEST_MAX_BUFFER_SIZE  1024*1024
result = WriteFile(hFile, buf, BufSize, &writeSize, NULL);

MB as expected = 1024 2 bytes.

h2testw - the program is free, no source code was found, I had to dissect:

And in this program the expected megabyte block is 1024 2 Bytes.

As a result of the difference in the block size to correctly classify memory card need to add the measured write speed approximately 5% (1024 2 /1000 2 = 1.049)


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