Storage array subsystems need code upgrades as fixes, patches, or feature upgrades. You can perform these upgrades online when the file system is mounted and I/Os are being served to the storage.
Storage subsystems contain multiple controllers for redundancy. An online upgrade is done one controller at a time. Dynamic Multi-Pathing (DMP) fails over all I/O to an alternate controller while one of the controllers is undergoing an Online Controller Upgrade. After the controller has completely staged the code, it reboots, resets, and comes online with the new version of the code. The other controller goes through the same process, and I/O fails over to the alternate controller.
Array vendors have different names for this process. For example, EMC calls it a nondisruptive upgrade (NDU) for CLARiiON arrays.
A/A type arrays require no special handling during this online upgrade process. For A/P, A/PF, and ALUA type arrays, DMP performs array-specific handling through vendor-specific array policy modules (APMs) during an online controller code upgrade.
When a controller resets and reboots during a code upgrade, DMP detects this state through the SCSI status. DMP immediately fails over all I/O to the next controller.
If the array does not fully support NDU, all paths to the controllers may be unavailable for I/O for a short period of time. Before beginning the upgrade, set the dmp_lun_retry_timeout tunable to a period greater than the time that you expect the controllers to be unavailable for I/O. DMP does not fail the I/Os until the end of the dmp_lun_retry_timeout period, or until the I/O succeeds, whichever happens first. Therefore, you can perform the firmware upgrade without interrupting the application I/Os.
For example, if you expect the paths to be unavailable for I/O for 300 seconds, use the following command:
# vxdmpadm settune dmp_lun_retry_timeout=300
DMP does not fail the I/Os for 300 seconds, or until the I/O succeeds.
To verify which arrays support Online Controller Upgrade or NDU, see the hardware compatibility list (HCL) at the following URL: