SnapVX can act on entire SGs. This reduces complexity by eliminating the need for users to create separate device files, device groups, or composite groups for replication. Those options are still available in Solutions Enabler. The SG option is the only option for SnapVX in Unisphere. This document focuses on the SG option in the following implementation section.

When creating snapshots with the symsnapvx establish command, the –sg switch is used to specify the source SG, and one snapshot of the same name is created for each volume in the SG. If for some reason a snapshot cannot be created for one of the volumes in the SG, the entire operation fails, and no snapshots are created.

When linking target volumes to a snapshot with the symsnapvx link command, the –lnsg switch is used to specify the entire target SG to link. Like the establish command, if any volume in the SG cannot be linked for some reason, the entire operation fails, and no volumes are linked. Unisphere also has an option that creates a new SG and appropriate volumes for the user.

In the case of cascaded SG, a symsnapvx command issued to a parent SG is applied to all child SGs.

Note: Solutions Enabler continues support of Composite Groups (-cg), Device Groups (-g) and Device Files (-f) options. Composite Groups provide the ability to take consistent snapshots across a set of devices that span multiple arrays.

SnapVX also introduces the –devs option to specify device range within the command line.

Expanding storage groups with active snapshots

SnapVX is designed to replicate entire applications that are configured on volumes contained in an entire SG. It is possible to add volumes to an SG that has active snapshots. The new volumes are included the next time a snapshot is taken. However, the volumes are not included in previously existing snapshots.

If an SG is restored from a snapshot that was taken before the new volumes were added to the SG, the SG is taken back to a point-in-time when the new volumes did not exist. Therefore, these new volumes in the SG should not be presented to the application and are set to not ready. The volumes remain not ready after the restored session is terminated and the user must decide the best course of action to reintroduce the new volumes to the SG. The volumes automatically become ready again if the user decides to restore from snapshots that include these volumes.

Similar behavior applies for linked target SGs that have been expanded. If the target SG is relinked to an older generation of the snapshot that was created before the additional volumes were added, the extra volumes in the target SG are set to not ready and unlinked from the newer snap. The user must decide the best course of action to remove or reintroduce the extra volumes. The volumes remain not ready if the unlinked. But if the SG is relinked to a generation of the snapshot that contains all volumes in the SG, the volumes are automatically be made ready and included in the session.