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Article Number: 000022742


Dell EMC Unity: How LUN trespassing takes place in Unity array. (User Correctable)

Summary: How does LUN trespassing in EMC Unity Array take place

Article Content


Instructions

The LUN trespass operation can happen due to several reasons, however from Unity Side a LUN trespass trigger occurs on either failure events for one of the SPs or a host using its non-optimized designed path . The owned LUNs are trespassed to the other SP.

In fact, Unity leverages Asymmetric Logic Unit Access (ALUA) for host access. This enables multiple paths to be active for I/O, but some paths are optimized while others are non-optimized. The paths that are designated as optimized are the ones connected to the SP that owns that particular resource. If I/O is sent down the non-optimized path, the peer SP leverages the internal CMI (Common Messaging Interface) bus to redirect the I/O to the SP that owns the resource. This operation is completely transparent to the host and removes the need to trespass the resource to complete the I/O. This all assumes that Best Practice for connecting a host has been followed - two HBA's per host, two switched and two SP ports per SP. This is configured for four paths - see second image below.
 
Example: Here are different scenarios for LUNs being trespassed to the other SP "initially un-owned one"
 
Scenario1 :
In the below setup, if SPA is down, its owned LUNs are trespassed from SPA to SPB till SPA is UP,  then the ownership of the LUNs are redirected back to the initial owner -  Trespassed back to SPA 
  kA5j0000000PBTeCAO_1_0
Scenario2:
If a large number of I/Os are received down the non-optimized path, the Unity system trespasses the LUN to the peer SP in order to optimize the data path.
The later mentioned scenario will happen when by error the Host will send its data on a wrong path so accordingly Unity will automatically trespass the LUNs to optimize the data path. 
 
So this is just a Unity defending mechanism to avoid high utilization of a SP that doesn t own a certain LUN.

Scenario3 :
In the below setup, In case of SP failure, the LUN fails over to the surviving SP and continues to service I/O since it is connected to the same switches. In case of switch failure, the remaining switch provides access to both SPs, eliminating the need to use the non-optimized path. In case of host failure, the cluster initiates a failover to the other host and brings the application online. Any path failure due to a bad cable or port does not cause any issues since the second optimized path can be used.
 
This configuration can also survive multiple failures, as long as they are not within the same component. For example, failure of Host B, Switch A, and SPA can be tolerated since the surviving components can be used to access the LUN. In this case, Host A can connect through Switch B, and access the LUN that s trespassed to SPB.
 
  kA5j0000000PBTeCAO_1_1

 

Article Properties


Affected Product

Dell EMC Unity Family

Product

Dell EMC Unity Family

Last Published Date

23 Nov 2020

Version

2

Article Type

How To