For array LUN aggregates, SyncMirror creates two physically separated copies of the aggregate, just as it does for disks.
These copies of the aggregate, called plexes, are simultaneously updated; therefore, the two copies of the data are always identical. Data continues to be served if one copy becomes unavailable.
For array LUNs, the physical separation of the plexes protects against data loss if the following occurs:
For example, a LUN failure can occur because of a double disk failure on the storage array.
An entire site could fail because of a disaster or prolonged power failure. If this situation occurs, the site administrator enters a command to enable the surviving node to take over the functions of the partner. Data is accessed on the plex of the surviving node.
Each of the two plexes must be on a separate set of array LUNs. The plexes can be in two physically separate locations on the same storage array, or each of the two plexes can be on a different storage array.
In a MetroCluster configuration with Data ONTAP systems that use array LUNs, each plex must be on a separate set of LUNs on different storage arrays. For MetroCluster configurations with Data ONTAP systems that use both array LUNs and disks, the plexes for disks are separate from the plexes for array LUNs.
Data ONTAP needs to know whether a plex is local to the system on which the aggregate is configured or in a remote location. "Local" in the context of storage arrays means on the storage array connected to the Data ONTAP systems on which the aggregate is configured. The SyncMirror pool to which an array LUN is assigned provides the information that Data ONTAP needs to determine whether the plex is local or remote.
The following illustration shows the relationships of plexes and pools to an aggregate. One plex is associated with pool0 and one plex is associated with pool1. The number 0 is typically associated with the local pool and the number 1 is typically associated with the remote pool. The remote plex is the mirror of the aggregate.