Solid-state drive (SSD) configuration rules

You must follow the SAN Volume Controller configuration rules for solid-state drives (SSDs).

Optional solid-state drives (SSDs) provide high-speed managed disk (MDisk) capability for SAN Volume Controller 2145-CF8 nodes. Each SAN Volume Controller 2145-CF8 node supports up to four SSDs. SSDs are local drives and are not accessible over the SAN fabric.

SSD configuration rules for nodes, I/O groups, and clusters

You must follow the SAN Volume Controller SSD configuration rules for nodes, I/O groups, and clusters:

Nodes that contain SSDs can coexist in a single SAN Volume Controller cluster with any other supported nodes.
Do not combine nodes that contain SSDs and nodes that do not contain SSDs in a single I/O group. However, while upgrading an earlier SAN Volume Controller node to a SAN Volume Controller 2145-CF8 node, you can temporarily combine the two node types in a single I/O group.
Nodes in the same I/O group must share the same number of SSDs.
Quorum functionality is not supported on SSDs within SAN Volume Controller nodes.

SSD configuration rules for MDisks and MDisk groups

You must follow the SAN Volume Controller SSD configuration rules for MDisks and MDisk groups:

Each SSD is recognized by the cluster as a single MDisk.
For each node that contains SSDs, create a single MDisk group that includes only the SSDs that are installed in that node.

SSD configuration rules for VDisks

You must follow the SAN Volume Controller SSD configuration rules for VDisks that use storage from SSDs within SAN Volume Controller nodes. In the following rules, SAN Volume Controller SSD storage is a managed disk group that uses SSDs within a SAN Volume Controller node.

Note: SSD storage within SAN-attached storage systems, such as the IBM® DS8000®, is not subject to these configuration rules.

VDisks that use SAN Volume Controller SSD storage must be created in the I/O group that the SSDs physically reside in.
VDisks that use SAN Volume Controller SSD storage must be mirrored to another managed disk group to provide fault tolerance. The following mirroring configurations are supported:
- To maximize performance, create the two VDisk copies in the two MDisk groups that correspond to the SAN Volume Controller SSD storage in two nodes in the same I/O group.
- To maximize utilization of SSD capacity, place the primary VDisk copy on SAN Volume Controller SSD storage, and the secondary copy on Tier 1 storage such as an IBM DS8000.
  Notes on capacity mirroring configuration:
  1. Under certain failure scenarios, VDisk performance degrades to the performance of non-SSD storage.
  2. All read I/O operations are sent to the primary copy of a mirrored VDisk, so read operations match SSD storage performance. Write I/O operations are mirrored to both locations, so write operations match the performance of the slowest copy.
To balance the read workload, evenly split the primary and secondary VDisk copies on each node that contains SSDs.
The preferred node for the VDisk must be the node that contains the SSDs that are used by the primary VDisk copy.
If you shut down a node that contains unmirrored VDisks that use SAN Volume Controller SSD storage, you will lose access to any VDisks that are associated with SSD storage in that node.
I/O requests to SSDs in other nodes are automatically forwarded, but this produces additional delays. The SSD configuration rules are designed to direct all host I/O operations to the node that contains the relevant SSDs.