transfer rate of dd to a disk is much less than copy file

Question

The case is that I use dd command to build a bootable usb for xxOS. /dev/disk2 is usb device. command is

dd if=xxOS.iso of=/dev/disk2 bs=8m

It spends a lot of time, the transfer rate is about 2MB/s~4MB/s. It makes me puzzle, why it's more slowly than copying files to an USB about 10MB/s or more up. May I use the right parameter?

Did you read the documentation of [dd(1)](https://man7.org/linux/man-pages/man1/dd.1.html) ? You could be interested in passing `oflags=sync` and `status=progress` — Basile Starynkevitch, Sep 23 '20 at 08:12

score 1 · Accepted Answer · answered Sep 23 '20 at 08:21

Don't assume that that the time taken for dd to return to the prompt is an accurate measure of the time taken to write the data to storage -- regardless of the device. Modern operating systems cache disk data in many different places. Running dd with the oflags=sync option will give a better idea of the real time, as might simply running sync at the prompt after the dd operation.

In general, though, storage I/O is pretty hard to time accurately on modern Linux systems.

In MacOS, use /dev/rdisk2 instead of /dev/disk2 will be the same. — ElapsedSoul, Sep 23 '20 at 12:09

transfer rate of dd to a disk is much less than copy file

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