A plex break-off snapshot uses an additional mirror to create the snapshot. Although you can create a plex break-off snapshot for a single plex volume, typically you take a snapshot of a mirrored volume. A mirrored volume has more than one plex or mirror, each of which is a copy of the data. The snapshot operation "breaks off" the plex, which becomes the snapshot volume. You can break off an existing plex or add a new plex specifically to serve as the snapshot mirror. Generally, you want to maintain redundancy for the original volume. If the original volume is a mirrored volume with two plexes, you add a third mirror for the snapshot. Hence, this type of snapshot is also known as a third-mirror snapshot.

The snapshot plex must be on a different disk from the existing plexes in the volume, within the same disk group. The disk must have enough disk space to contain the contents of the existing volume. If you have a one terabyte volume, you must have an additional one terabyte of disk space.

When you create the snapshot, the plexes are separated into two volumes. The original volume retains its original plex or plexes. The snapshot volume contains the snapshot plex. The original volume continues to take on I/O. The snapshot volume retains the data at the point of time when the snapshot was created, until you choose to perform processing on that volume.

You can make multiple snapshots, so you can have multiple copies of the original data.

Third-mirror break-off snapshots are suitable for write-intensive volumes (such as for database redo logs) where the copy-on-write mechanism of space-optimized or full-sized instant snapshots might degrade performance.