11.2. Creating snapshot objects

You can create a snapshot object from any unused storage object in EVMS (disks, segments, regions, or feature objects). The size of this consumed object is the size available to the snapshot object. The snapshot object can be smaller or larger than the original volume. If the object is smaller, the snapshot volume could fill up as data is copied from the original to the snapshot, given sufficient activity on the original. In this situation, the snapshot is deactivated and additional I/O to the snapshot fails.

Base the size of the snapshot object on the amount of activity that is likely to take place on the original during the lifetime of the snapshot. The more changes that occur on the original and the longer the snapshot is expected to remain active, the larger the snapshot object should be. Clearly, determining this calculation is not simple and requires trial and error to determine the correct snapshot object size to use for a particular situation. The goal is to create a snapshot object large enough to prevent the shapshot from being deactivated if it fills up, yet small enough to not waste disk space. If the snapshot object is the same size as the original volume, or a little larger, to account for the snapshot mapping tables, the snapshot is never deactivated.

After you've created the snapshot object and saved the changes, the snapshot will be activated (as long as the snapshot child object is already active). This is a change from snapshots in EVMS 2.3.x and earlier, where the snapshot would not be activated until the object was made into an EVMS volume. If you wish to have an inactive snapshot, please add the name of the snapshot object to the "activate.exclude" line in the EVMS configuration file (see section about selective-activation for more details). If at any point you decide to deactivate a snapshot object while the original volume is still active, the snapshot will be reset. The next time that the snapshot object is activated, it will reflect the state of the original volume at that point in time, just as if the snapshot had just been created.

In order to mount the snapshot, the snapshot object must still be made into an EVMS volume. The name of this volume can be the same as or different than the name of the snapshot object.