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Re: Unusable system during dd to USB block device

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.


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