While dding a CentOS 7 x86_64 DVD image to a USB drive (sdb), I monitored the progress with iostat.It kept alternating between mostly writing to the USB drive:Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtnsda              18,00       380,00        28,00        380         28sdb              79,00        16,00      9000,00         16       9000And reading from it hard disk and USB drive simultaneously:Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtnsda              53,00      5180,00         0,00       5180          0sdb            1329,00      5316,00         0,00       5316          0There was nothing else going on in the machine and, when dd stopped, the disks became idle.Is it some sort of internal checksumming process in dd? Anything inherent to USB drives' drivers in Linux 3.16.0?
Why dd reads from output device?
dd