Snapshots (virtual copies)

HPE SSMC allows you to instantly create point-in-time snapshots (virtual copies) of virtual volumes.

At the point in time when a snapshot is created, it is instantly available for use and contains only pointers to the source virtual volume. As data on the source virtual volume changes, the changed data is recorded in the snapshot. The HPE Storage OS uses the pointers and the tracked changes to represent the original source virtual volume.

IMPORTANT:

If more than the advisable number of snapshots runs around the same time, they utilize the system manager resources and system I/O bandwidth, which slows the system down.

Hewlett Packard Enterprise recommends to limit the number of schedules to the following numbers for optimal storage system performance:

  • Up to 375 schedules for HPE 3PAR 7200 and HPE 3PAR 7400.

  • Up to 500 for HPE Primera storage systems and HPE 3PAR 8000, 9450, 10000, and 20000 models.

The data on a snapshot is consistent with its source but is not necessarily consistent with the host file system or application. That is, a snapshot preserves the data that was written to the source virtual volume when the snapshot was created, but it does not preserve data that resides only in host application memory or file system buffers.

  • Snapshots can be made of base virtual volumes, clones, or other snapshots.

  • The number of snapshots per base virtual volume that can be made varies with the version of HPE Storage OS.

  • The maximum number of snapshots per storage system depends on the storage system configuration.

Creation and management of virtual volume snapshots on a storage system requires an HPE 3PAR Virtual Copy software license.

Read and write modes

The relationship of read and write modes for a source virtual volume and its snapshots is based on the following:

  • Base virtual volumes are always read/write mode.

  • A snapshot of a base virtual volume is always read-only mode.

  • Subsequent snapshots (on the same tree) alternate between read/write and read-only modes.

Snapshot promotion

Snapshot promotion copies data from a snapshot back to the source base virtual volume or to a parent read/write snapshot on the same tree. The feature allows a base virtual volume or snapshot to be rolled back to a previous point in time. The promoted snapshot remains on the system.

  • A snapshot cannot be promoted if it is exported to a host or the target virtual volume is exported to a host.