How do I add raw device mapping in Red Hat Enterprise Linux 5

Issue

How do I add raw device mapping in Red Hat Enterprise Linux 5?

Environment

• Red Hat Enterprise Linux 5
• raw device

Resolution

Previously (Red Hat Enterprise Linux 5.0 through Red Hat Enterprise Linux 5.3), support for raw devices in the upstream kernel was deprecated. However, this support has been returned to the kernel. Consequently, in Red Hat Enterprise Linux 5.4, support for raw devices has also been returned. Additionally, the initscripts packages have been updated, adding the previously dropped functionality of raw devices.

So in the Red Hat Enterprise Linux 5, there are two methods to configure RAW device.

Method 1. Using rawdevices service (Not available on RHEL5.0 -- RHEL5.3)

1. Edit the file /etc/sysconfig/rawdevices

    service rawdevices start

    # raw device bindings

    # format:  <rawdev> <major> <minor>

    #          <rawdev> <blockdev>

    # example: /dev/raw/raw1 /dev/sda1

    #          /dev/raw/raw2 8 5

    /dev/raw/raw1 /dev/hda5

    /dev/raw/raw2 /dev/hda6

(Notes: /dev/raw/raw0 is not allowed because minor number cannot be zero.)

2. Start the rawdevices service

    #service rawdevices start

    #chkconfig rawdevices on

Method 2. Using udev to configure RAW device

1. Creating the raw devices using udev:

Nevertheless, to create raw devices, add entries to /etc/udev/rules.d/60-raw.rules in the following formats:

For device names:

ACTION=="add", KERNEL=="<device name>", RUN+="raw /dev/raw/rawX %N"

For major / minor numbers:

ACTION=="add", ENV{MAJOR}=="A", ENV{MINOR}=="B", RUN+="raw /dev/raw/rawX %M %m"

Note: Replace <device name> with the name of the device needed to bind (such as /dev/sda1). "A" and "B" are the major / minor numbers of the device needed for binding, an "X" is the raw device number that the system wants to use.

2. Creating persistent raw devices for single path LUNs: 
If using unpartitioned LUNs, to create a single raw device for the whole LUN use this rule format:

ACTION=="add", KERNEL=="sd*[!0-9]", PROGRAM=="/sbin/scsi_id -g -u -s %p", RESULT=="3600601601bd2180072193a9242c3dc11", RUN+="/bin/raw /dev/raw/raw1 %N"

Note: Set the RESULT value to the output of scsi_id -g -u -s /block/sdX (where sdX is the current path to the LUN). This will create the raw device /dev/raw/raw1 that will be persistently bound to the LUN with WWID 3600601601bd2180072193a9242c3dc11.

If using partitioned LUNs, where raw devices are created for each of the partitions on the LUN, use this rule format:

 ACTION=="add", KERNEL=="sd*[0-9]", PROGRAM=="/sbin/scsi_id -g -u -s %p", RESULT=="3600601601bd2180072193a9242c3dc11", RUN+="/bin/raw /dev/raw/raw%n %N"

Again

, set RESULT to the output of scsi_id -g -u -s /block/sdX. This will create the raw device(s) /dev/raw/raw1, /dev/raw/raw2, etc. for each partition on the LUN and they will be persistently bound to the LUN with WWID 3600601601bd2180072193a9242c3dc11.

3. Setting ownership and permissions on the raw devices: 

To set specific ownership and/or permissions for the raw devices, add entries to /etc/udev/rules.d/60-raw.rules in the following format:

 ACTION=="add", KERNEL=="raw*", OWNER="root", GROUP="disk", MODE="0660"

4. Testing and implementing the udev rules: 

Before implementing them, use the udevtest command to verify the udev rules work as expected. To verify that the raw device is created for a specific disk or partition, eg /dev/sdb1:

 [root@rhel5 rules.d]# udevtest /block/sdb/sdb1 | grep raw

 main: run: '/bin/raw /dev/raw/raw1 /dev/.tmp-8-17'

To check ownership/permission settings for a particular raw device, eg /dev/raw/raw1:

 [root@rhel5 rules.d]# udevtest /class/raw/raw1 | grep mode

 udev_node_add: creating device node '/dev/raw/raw1', major = '162', minor = '1', mode = '0600', uid = '0', gid = '0'

Finally, to actually create the raw device(s), use the start_udev command:

 [root@rhel5 rules.d]# start_udev

 Starting udev:                                             [  OK  ]

Check that the raw device(s) have been created:

 [root@rhel5 rules.d]# raw -qa

 /dev/raw/raw1:  bound to major 8, minor 17

 [root@rhel5 rules.d]# ls -l /dev/raw

 total 0

 crw-rw---- 1 root   disk 162,  1 Jan 29 02:47 raw1

5. Creating persistent raw devices for multipathed LUNs or LVM device: 

Unfortunately it is not possible to write udev rules for creating raw devices on multipath devices (/dev/dm-*) without manipulating existing udev rules. Modifying existing rules for this purpose could cause unforeseen problems and is not supported by Red Hat Global Support Services. If absolutely necessary, an alternate method for creating raw devices on top of multipath devices could be to create the raw devices in /etc/rc.d/rc.local, so long as the raw device is not required before rc.local is executed. For example:

 /bin/raw /dev/raw/raw1 /dev/mpath/mpath1p1

 /bin/raw /dev/raw/raw2 /dev/mpath/mpath1p2

 /bin/chmod 660 /dev/raw/raw1

 /bin/chown root:disk /dev/raw/raw1

 /bin/chmod 660 /dev/raw/raw2

 /bin/chown root:disk /dev/raw/raw2

If absolutely want to create raw devices for multipathed LUNs or LVM device using udev, can add the following udev rules in the file /etc/udev/rules.d/60-raw.rules

# Device mapper raw rules

KERNEL!="dm-[0-9]*", GOTO="skip_dm"

ACTION!="change", GOTO="skip_dm"

PROGRAM!="/sbin/dmsetup ls --exec /bin/basename -j %M -m %m", GOTO="skip_dm"

RESULT=="mpath2", RUN+="/bin/raw /dev/raw/raw2 /dev/mapper/mpath2"

RESULT=="mpath1", RUN+="/bin/raw /dev/raw/raw1 /dev/mapper/mpath1"

LABEL="skip_dm"

KERNEL=="raw1", ACTION=="add", OWNER="root", GROUP="disk", MODE="0660"

KERNEL=="raw2", ACTION=="add", OWNER="root", GROUP="disk", MODE="0660"

Note: The first three rules makes sure we are only using dm devices with change actions, when dmsetup returns success. If not, we skip down to the "skip_dm" label.  After that, the rules look at the result of running dmsetup, and do fire off the appropriate command.