MPIO with MSDSM requires symmetric disk presentation

Applies to: Microsoft Windows Server 2003 Datacenter Edition (32-bit x86)Microsoft Windows Server 2003 Datacenter Edition for Itanium-Based SystemsMicrosoft Windows Server 2003 Datacenter x64 Edition

Summary


MPIO with MSDSM (Microsoft Device Specific Module) has an implicit requirement for symmetric presentation of disks across logical paths.

If a disk is presented on a given logical path, all disks presented on that path need to also be presented on all logical paths that contain any subset of those disks.

Logical paths correspond to LUN-path pairs which are managed by the DSM.  Physical paths correspond to the actual physical hardware over which the logical paths are hosted.  There may be more logical paths than physical paths, for example, with multiple logins in iSCSI scenarios, or in switched fibre channel infrastructure scenarios.

Invalid configurations may result in performance issues and spurious disk or path failures during failover scenarios.
 
For example:
Consider the following configuration:
- Disks A, B, and C 
- Paths 1, 2, 3, and 4

If disks A and B are both presented on paths 1, 2, and 3, and disk C is presented to only path 4, this is a valid configuration as it meets the above requirement of symmetrical presentation.

Valid
Path 1: A, B
Path 2: A, B
Path 3: A, B
Path 4: C
 
By contrast, if disk A and B are both presented on path 1, disk B is presented on path 2, disk A is presented on path 3, and disk C is presented on path 4, this is an invalid configuration because paths 2 and 3 are not symmetric with path 1.

Invalid
Path 1: A, B
Path 2: B
Path 3: A
Path 4: C

To confirm symmetric disk presentation, use Device Manager to compare the Path ID assigned to each disk (LUN or Logical Unit Number presented by the SAN) as described in the following steps:

1. Open Control Panel and launch Device Manager.

2. Select View, Devices by Connection.

3. Select each disk device under the Microsoft Multi-Path Bus Driver and select Properties from right click menu.

4. In the Properties window, select the General tab and record the LUN number indicated by the Location. Then go to the MPIO tab and record each Path ID for each disk device in step 3.

For example, you may find the following:

LUN1 is presented on Path ID 77030000 and 77030001. 
LUN2 is presented on Path ID 77030000 and 77030001. 
LUN3 is presented on Path ID 77030000 and 77030001. 
LUN4 is presented on Path ID 77030002.

Because each LUN that is presented to Path ID 77030000 is also presented to Path ID 77030001 this is a symmetric presentation. Although LUN4 is only presented to Path ID 77030002 it is symmetric because there are no other LUNs presented to that Path ID.

However if you were to find the following:

LUN1 is presented on Path ID 77030000 and 77030001. 
LUN2 is presented on Path ID 77030000.
LUN3 is presented on Path ID 77030001. 
LUN4 is presented on Path ID 77030002. 

It would be considered an asymmetric presentation because LUN1 was presented to both Path ID's but LUN2 was only presented to path 77030000 and LUN3 was only presented to 77030001. To correct this invalid configuration, modify the LUN2 to add Path ID 77030001 and modify LUN3 to add Path ID 77030000. No change is required to the LUN4 presentation since no other LUNs are presented to that Path ID.