Adjusting the Disk File Read-Ahead Parameter

Principle

The principle of file prefetching is to read a certain amount of adjacent data and cache them on the memory when reading data based on the locality principle. If the read-ahead data is subsequently used, the system performance will be improved. If the pre-read data is not subsequently used, the disk bandwidth will be wasted. In sequential disk read scenarios, increasing the prefetch value is especially effective.

Procedure

The file prefetch parameter is specified by the read_ahead_kb file. In CentOS, the file is in the /sys/block/$DEVICE-NAME/queue/read_ahead_kb directory ($DEVICE-NAME indicates the disk name).

If you are not sure about the disk name, run the following command to find the file.

find / -name read_ahead_kb

The default value of this parameter is 128 KB. You can run the echo command to change the value. The following command changes the prefetch value to 4096 KB, assuming CentOS is used:

echo 4096 > /sys/block/$DEVICE-NAME/queue/read_ahead_kb