Red Hat Training

A Red Hat training course is available for Red Hat Enterprise Linux

13.2. Basics of Raw Devices

To bind the first raw device /dev/raw/raw1 to the /dev/sdz SCSI disk or LUN you can execute the following command:
# raw /dev/raw/raw1 /dev/sdz
Now when you run the dd command on /dev/raw/raw1, it will write directly to /dev/sdz bypassing the OS block buffer cache:


The following command will overwrite data on dev/sdz
# dd if=/dev/zero of=/dev/sdz count=1
To permanently bind /dev/raw/raw1 to /dev/sdz, add an entry to the /etc/sysconfig/rawdevices file:
/dev/raw/raw1 /dev/sdz
Now when you run /etc/init.d/rawdevices it will read the /etc/sysconfig/rawdevices file and execute the raw command for each entry:
/etc/init.d/rawdevices start
To have /etc/init.d/rawdevices run each time the system boot, it can be activated by executing the following command:
chkconfig rawdevices on


For each block device you need to use another raw device.
To bind the third raw device to the second partition of /dev/sdz, the entry in /etc/sysconfig/rawdevices would look like this:
/dev/raw/raw3 /dev/sdz2
Or to bind the 100th raw device to /dev/sdz, the entry in /etc/sysconfig/rawdevices would look like this:
/dev/raw/raw100 /dev/sdz