Failure group

Shared Storage Pool (SSP) Mirroring is enabled from Virtual I/O Server (VIOS) Version 2.2.3. Mirroring an SSP is an optional step that increases resiliency by adding redundancy. Inside the storage pool, there might be two sets of shared logical unit numbers (LUNs, or physical volumes (PVs)). These two named sets of LUNs are referred to as failure groups or mirrors. The preferred practice is to define the two failure groups on different physical storage arrays for best availability.

The whole pool is either a single copy pool (one failure group) or double copy pool (two failure groups). If two groups are defined, the whole pool is mirrored and not just individual logical units (LUs) of PVs. The data space that belongs to an LU is divided into 64 MB each and the LUs are placed in individual physical volumes (LUNs) in the pool. The exact data placement is decided in the background. Hence, it is not an exact one-to-one mirroring.

By default, a single copy pool is created by running the cluster -create command and the first failure group is named Default. You can rename the first failure group and add a second failure group.

Consider the following characteristics of a mirrored SSP:
  • A mirrored SSP doubles the disk space requirement, which is typical for Disaster Recovery (DR) solutions.
  • A mirrored SSP is completely transparent for client VMs. Therefore, there is no action needed on the client operating system. The VIOS accesses the storage and keeps the mirrors in a synchronized state. The VIOS creates duplicates, writes to both mirrors and performs re-mirroring if one of the mirrors becomes out-of-sync.
  • The VIOS performs recovery and re-mirroring in the background, without affecting the client VMs.
The following preferred practices relate to mirrored storage pools:
  • Failure groups must be of the same size. If there are two failure groups in an SSP and their capacity is not the same, the total size of the SSP available for allocation of LUs is the sum of the capacity of LUNs that are in the smaller failure group. The rest of the capacity in the larger failure group is not used.
  • When you create a large mirrored pool with two failure groups, the preferred practice is to create a pool of one disk and add the second failure group to mirror the first pool. Then, you can add physical volumes to both failure groups to increase the capacity of the pool.
  • If a disk or a storage controller in a single failure group fails, the mirrored storage pool runs in a degraded state. In this case, you must take corrective actions to resolve the issue on the storage controller.
  • The system firmware must be upgraded to the latest release to achieve the optimum performance from the mirrored storage pools.