On Mon, 28 Dec 2020, Arto Huusko wrote:
I just accidentally tried to write about 3 gigabytes of data with dd to USB disk via the block device instead of raw device. The transfer did work, but it was excruciatingly slow, under 2 Mb / sec. But that wasn't the only problem: it also made most other activities on the system unusable, at least if there was any disk access involved.
This is why there is a raw device--to bypass the buffer cache. When you use dd, write to the raw device. Or pass `iflag=direct' and `oflag=direct' if you use the normal disk device. Linux needs those, otherwise dd will fill up all available buffers there too. -RVP