An AIX host is connected to a Huawei Symantec storage device. The #dd if=/dev/updisk0 of=/dev/null bs=1024K command is executed to test the system performance. The bandwidth is only 30 Mbit/s. # dd if=/dev/updisk0 of=/dev/null bs=1024K The performance is lower than the expected performance. The test cannot be passed on the customer’s site.
It is recommended to use the character devices to test the I/O requests larger than 4 Kbit/s but the block devices to I/O requests smaller than 4 Kbit/s. The block device and character device based on the same disk respectively correspond to the dd if=/dev/updisk0 of=/dev/null bs=1024K and dd if=/dev/rupdisk0 of=/dev/null bs=1024K commands. To meet customers’ requirements on performance, use character devices to test product performance.
Block devices support up to 4 Kbit/s I/O requests. The block device layer splits the I/O requests larger than 4 Kbit/s and combines those smaller than 4 Kbit/s.
It is recommended to use character devices to test product performance according to customers' test environment.