When upgrading SAN Volume Controller clusters that contain 2145-CF8 nodes with solid-state drives, the concurrent code upgrade might not complete successfully if the synchronisation rate of any mirrored VDisk is set too low.
If a cluster contains 2145-CF8 nodes that include the optional solid-state drives (SSDs), you must use VDisk mirroring to maintain a copy of the data on an SSD in a different node, or on external SAN disks. During a software upgrade, SAN Volume Controller nodes will go offline as part of the upgrade process and all VDisk copies that are stored on SSDs in that node will go offline. Once the node comes back online, the VDisk mirroring function will automatically synchronise the VDisk copies.
The rate of synchronisation is controlled by the sync rate property of the VDisk, which can be modified by a user. Testing has shown that when using the default sync rate of 50, the copy of data kept on the first node to be updated might not become fully synchronised before the second node is scheduled to be upgraded. If the SVC cluster detects that the VDisk mirrors using the SSDs are not synchronised, the upgrade process will stall and the upgrade will have to be completed under guidance from IBM support.
Resolving the problem
To minimize the risk of not fully re-synchronising the VDisk mirrors during an upgrade, set the sync rate for all VDisks that are mirrored to a solid state drive to 80 or above for the duration of the upgrade.
The latest SVC upgrade test utility will warn of any VDisks that are affected by this issue.