Planning for LVM split-site mirroring

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You can set up disks located at two or three different locations for remote Logical Volume Manager (LVM) mirroring by using a storage area network (SAN). For example, split-site mirroring uses LVM to replicate data between the disk subsystem at each different location for disaster recovery.

Note: PowerHA® SystemMirror® only supports two site configurations.

A SAN is a high-speed network that allows your environment to establish direct connections between storage devices and systems (nodes). Thus, two or more systems located at different locations can access the same physical disks by using a SAN network connection. You can combine remote disks into a volume group using LVM. You can import this volume group to the nodes located at different locations.

The logical volumes in the volume group that contain the remote disks can have up to three remote mirrors. You can set up at least one remote mirror at each location. The data stored in the logical volume is highly available. Therefore, in case of a failure in your environment, for example all nodes are not available at one location, the remote mirror at another location has the latest data.

PowerHA SystemMirror automatically synchronizes all remote mirrors after a disk or node failure occurs, and the nodes are brought back online. Automatic synchronization happens even if one of the disks is in the PVREMOVED state or the PVMISSING state. Automatic synchronization is not available for all cases of LVM split-site mirroring. If it is not available, you can use C-SPOC to synchronize the data.

When planning for an LVM split-site mirroring configuration, you must also plan for the repository disk used in the cluster. You must verify that there is a second disk ready to be used as a repository disk when the primary disk fails.
Note: Cluster Aware AIX® supports live repository replacement without impacting critical cluster functions.

Example

The following figure is a configuration example of LVM split-site mirroring using a SAN.
Figure 1. LVM split-site mirroring configuration using a SAN

Displays two environments at different locations that are using a LVM split-site mirroring configuration using a SAN.
You can mirror the disks that are connected to at least one node at each of the two locations. In this example, PV4 is available for Node A and Node B on Location 1 and Node C on Loaction 2 using the Fibre Channel Switch 1 and Fibre Channel Switch 2 connection. You can have a mirror of PV4 on Location 1. The disks that are connected to the nodes on only one location (PV5 and PV6) cannot be mirrored across locations.

You can use the AIX LVM mirrored pools function to ensure that data is correctly and completely mirrored between the two locations. If PV1 and PV2 are in one mirrored pool, and PV3 and PV4 are in a separate mirrored pool, then LVM allows one complete copy of the data to be present at each location. You must use superstrict mirrored pools to guarantee that a complete copy of the data is at each location.

In this example, you can use mirrored pools to help maintain mirroring even as disks are added, removed, or replaced in a volume group. You can also use the C-SPOC function to define mirrored pools and associate them with the disks at each location.