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Block Devices and Character Devices Impose an Effect on the Storage System Performance

Publication Date:  2012-07-18 Views:  76 Downloads:  0

Issue Description

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.

Alarm Information


Handling Process

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.

Root Cause

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.