PowerPath: How to configure with Oracle ASM for clustered Oracle RAC servers

Oracle Automatic Storage Management (ASM) ensures consistent naming of devices across Real Application Clusters (RAC). It also maintains permissions on devices across reboots. This feature was important until UDEV rules were added to Linux with the 2.5 kernel.

A feature of ASM is that devices stamped for ASM are assigned an alias. Each disk is added to the /dev/oracleasm/disks/ device directory, and presented to ASM using the alias ORCL:diskname.

When Linux sees multiple paths to the same disk or LUN, it creates an entry in the SCSI device table for each path. Therefore, a single LUN with two paths may appear to Linux as both /dev/sdg and /dev/sdh, for example.

This is problematic for ASM, since ASM cannot handle two or more devices mapping to the same LUN.

Like ASM, Dell PowerPath also creates aliases: these are our pseudo-devices (/dev/emcpowerX where X is the device letter).

Process
One of the main issues to address with RHEL servers in a cluster is that Dell pseudo names must match. The pseudo names are seen by running the powermt display dev=all command.
 

 
If the command is run on both hosts, notice that the pseudo names are different. 

An article found on the Internet suggests the following:   
 

"Shutdown PowerPath on one of the clustered nodes.

$ sudo /etc/init.d/PowerPath stop

Now that PowerPath is stopped, copy PowerPath configuration files from node one to node two. Copy /etc/emcp_devicesDB.dat and /etc/emcp_devicesDB.idx to the /etc directory on the node. Once the copy is complete, start PowerPath up (sudo /etc/init.d/PowerPath start) and run the powermt display dev=all command and the pseudo names will match."

Dell suggests using an "emcpadm export_mappings -f <pseudo_mapping>" on one node.
This saves the mapping between the array devices and the pseudo-devices.
Use "emcpadm import_mappings -f <pseudo_mappings>" on the other nodes.
<pseudo_mapping> is the file which contains the mapping between the pseudo-devices and the array devices.

Once this step is complete, update /etc/sysconfig/oracleasm. The directives ORACLEASM_SCANORDER and ORACLEASM_SCANEXCLUDE must be modified as follows:    
 

Now, when ASMLib starts, it scans for ASM disks among the PowerPath pseudo-devices instead of the SCSI devices.