A distinction is made between original unit and snap unit.
The original unit is generally in the READY status. It can be mirrored by RAID, SnapOPC+, EC or REC.
The snap unit cannot be mirrored. The relevant status of the snap unit is determined by the status of the snap session.
From the SHC-OSD viewpoint the lifetime of a snap session is the lifetime of a snap pair, i.e. it extends from the time it is created (/START-SNAP-SESSION) to the time it is terminated (/STOP-SNAP-SESSION).
During its lifetime, a snap session can assume one of the following statuses:
CREATE-IN-PROG
The snap session (the snap pair) currently being created and activated, i.e. the snap unit is currently being connected to the original unit.
The snap unit is in the NOT-READY status.
COPY-ON-WRITE
The snap session is activated. The original unit and snap unit can be processed separately. Only the original versions of modified data are copied onto the volumes provided for this purpose.
REST-IN-PROG
The original unit is reconstructed from the snap unit; the saved data is copied from the snap unit to the original unit.
The snap unit is in the READY status.
UNUSED
The snap unit is not connected to any original unit. It is free for a /START-SNAP-SESSION.
The snap unit is in the NOT-READY status.
FAILED
The snap session has become unusable (e.g. because of a save pool overflow); the data on the snap unit is lost. The session can only be terminated using /STOP-SNAP-SESSION.
The snap unit is in the NOT-READY status.