Using the PSP_RR path selection policy with MSCS results in quorum disk problems

Products

• VMware

Issue/Introduction

MSCS conducts SCSI-3 reservations on the quorum disk. SCSI-3 registration sent down one path allows the MSCS cluster to do SCSI-3 reservations only on that path. When PSP_RR later switches to another path, MSCS receives an error if it tries to do a reservation or use other SCSI-3 commands, depending on the reservation.

Symptoms:
When you use the PSP_RR path selection policy with MSCS, the quorum disk experiences problems and the cluster may not operate.

Resolution

This issue is resolved in vSphere 5.5. For more information about this version, see the VMware vSphere 5.5 Release Notes. You can download the latest release from the VMware Download Center.

With the introduction of round-robin support, there are a number of changes for the SCSI locking mechanism used by MSCS when a failover of services occurs. To facilitate this new path policy, these changes mean that it is irrelevant which path is used to place the SCSI reservation; any path can free the reservation.

For ESX versions 5.1 and earlier, do not switch the device used for the MSCS quorum disk to PSP_RR. Instead, use the PSP_MRU or PSP_FIXED policies depending on the normal default for the array.

ESXi uses Roundrobin policy (VMW_PSP_RR) for some storage arrays by default. If you use the LUN for MSCS it must be changed to use a different policy, as specified in this table:

Array	SATP	PSP to use for MSCS
EMC Clariion	ALUA_CX	FIXED
EMC Symmetrix	SYMM	FIXED
EMC VNX	ALUA_CX	FIXED
HITACHI	DEFAULT_AA	FIXED
IBM 2810XIV	ALUA	MRU
IBM 2810XIV	DEFAULT_AA	FIXED
NETAPP Data ONTAP 7-Mode	DEFAULT_AA	FIXED