IBM DS3500 Storage - Logical Drive not on preferred path due to ADT/RDAC failover

Problem Cause :-

It happens is that when a LUN is owned by a preferred controller and it failed over to the other controller, you get the error message. 

Explanation :- 

Lets explain about LUN fail over using a real life example. Say host A loses access to array controller port B0 (which is currently active & preferred across all hosts). It must then select a new path. It may try to select controller port B1 then, but if this is also dead(here the controller ports might be working and because of some slower response also hosts will select other path), it might select port A0 and move all its I/O over to that path. Since we are switching I/O to a different controller, this will cause a trespass where LUNs may have to move ownership from controller B to controller A. If controller B later becomes available, then host A can switch back to its original preferred path since it was the cause of the original failover. 

However other hosts, which had to move to controller A because of the actions taken by host A, cannot switch I/O back to controller B. Those other hosts do not know if host A can still access controller B (even if they can access it). But if host A can see the path to port B0 again, then it can try to switch back to the preferred path. 

On the other hand, if it was another host which lost access to controller B and they moved their active path to another port on controller A, this also implies that host A loses access to controller B as I/O has to move to the new active path on controller A. In this case host A cannot pull back the LUN to its preferred path on controller B, even if it has access to it. This is because host A did not initiate the failover and make the paths to controller B standby. 

-Trespass of Storage LUNs is common in any storage array and it depends on the internal SCSI algorithms within storage OS designed by the vendor due to storage path access issues. 

-However if you still want to move your logical drives back you can do so by using the storage manager by hitting the option under Advanced -> Recovery -> Redistribute Volumes. 

-This will set the storage array status to optimal and please do this in a less I/O transaction time and if this is not setting the LUN to its original preferred path we would need to get in touch with IBM. 

Solution :-

The message received mean the logical drive changed its preferred path 

intermittently.  What happens is that when a LUN is owned by a preferred controller and 

it failed over to the other controller, you get the error message. 

Manually change the preferred path for these volumes in the Storage manager using 

Change >> Ownership/Preferred Path or Logical Drive >> Change >> 

Ownership/Preferred Path menu options