Electronic – Is it normal for a micro SD card to have higher write speed for the first blocks

microsdsdsolid-state-devices

I'm using a micro SD card of 4Gb from Kingston in an embedded project and I'm accessing it with a low level SPI driver.

Everything is running just fine, except for a weird behavior: after a given block (around 16 Kb of the card), the write speed drops to 1/3 of the first block's speed.

Is this normal? Maybe the first blocks of an SD card are of a higher speed memory, I don't know. Does someone have info about this?

Best Answer

Yes, this is normal. Many SD cards have a write buffer (think of it like a disk cache). If you read the datasheet for the specific component it should detail the size of the cache and access speeds of the device in general.

If you think that the speed is based on the block the test is easy: write to the SD card at some offset past where you think the "fast blocks" are -- they should be just as quick for the first writes.