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33.14. Configuring LUN Persistence

This section covers how to implement LUN persistence in guests and on the host machine with and without multipath.
Implementing LUN persistence without multipath

If your system is not using multipath, you can use udev to implement LUN persistence. Before implementing LUN persistence in your system, ensure that you acquire the proper UUIDs. Once you acquire these, you can configure LUN persistence by editing the scsi_id file that resides in the /etc directory. Once you have this file open in a text editor, you must comment out this line: 

# options=-b
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Then replace it with this parameter: 
# options=-g
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This tells udev to monitor all system SCSI devices for returning UUIDs. To determine the system UUIDs, use the scsi_id command: 
# scsi_id -g -s /block/sdc
*3600a0b80001327510000015427b625e*
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The long string of characters in the output is the UUID. The UUID does not change when you add a new device to your system. Acquire the UUID for each device in order to create rules for the devices. To create new device rules, edit the 20-names.rules file in the /etc/udev/rules.d directory. The device naming rules follow this format: 
# KERNEL="sd*",  BUS="scsi",  PROGRAM="sbin/scsi_id", RESULT="UUID", NAME="devicename"
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Replace your existing UUID and devicename with the above UUID retrieved entry. The rule should resemble the following: 
KERNEL="sd*",  BUS="scsi",  PROGRAM="sbin/scsi_id", RESULT="3600a0b80001327510000015427b625e", NAME="mydevicename"
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This enables all devices that match the /dev/sd* pattern to inspect the given UUID. When it finds a matching device, it creates a device node called /dev/devicename. For this example, the device node is /dev/mydevice . Finally, append the /etc/rc.local file with this line: 
/sbin/start_udev
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Implementing LUN persistence with multipath

To implement LUN persistence in a multipath environment, you must define the alias names for the multipath devices. For this example, you must define four device aliases by editing the multipath.conf file that resides in the /etc/ directory: 

multipath  {  
             wwid       3600a0b80001327510000015427b625e
             alias      oramp1
}
multipath  {  
             wwid       3600a0b80001327510000015427b6
             alias      oramp2
}
multipath  {  
             wwid       3600a0b80001327510000015427b625e
             alias      oramp3
}
multipath  {  
             wwid       3600a0b80001327510000015427b625e
             alias      oramp4
}
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This defines 4 LUNs: /dev/mpath/oramp1, /dev/mpath/oramp2, /dev/mpath/oramp3, and dev/mpath/oramp4. The devices will reside in the /dev/mpath directory. These LUN names are persistent after reboots as it creates aliased names on the wwid for each of the LUNs.
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