At the start of a snap session, snap pairs are created from the original units and the free snap units using the /START-SNAP-SESSION command. The snap units must be free. For ETERNUS DX/AF this means that they must be in the UNUSED status from the point of view of snap unit usage.
The snap pairs can be created during normal processing of the original unit. Snap pairs on ETERNUS DX/AF are activated immediately when a command is executed.
Considerations on a consistency point and renaming should therefore be taken into account already at the creation of the clone pairs (see the respective descriptions in section "Synchronization point for replications").
A distinction must be made according to whether snap units are created for the entire pubset, only for one original unit or for more than one particular original unit.
Entire pubset
An appropriate snap unit, identified by its mnemonic name (MN), must be available for each original unit.
Create snap pairs for all original units of a pubset:
/START-SNAP-SESSION UNIT=*BY-PUBSET(PUBSET=<cat-id>) ,SNAP-UNIT=<snap units MN list> / *AUTO-SELECT
The sequence of the snap units is determined by the VSN of the pubset’s disks. If, for example, the pubset ID or volume set ID is SNP, the first snap unit is assigned to the VSN SNP.00, the second to the VSN SNP.01, and so forth.
SNAP-UNIT=*AUTO-SELECT provides a user-friendly way of assigning suitable snap units to a pubset. SHC-OSD automatically selects appropriate snap units depending on the storage system. In the case of automatic selection, only devices which are configured and attached in the local BS2000 are taken into consideration.
Individual original units
Create a snap pair with one original unit and one snap unit:
/START-SNAP-SESSION UNIT=<unit MN> ,SNAP-UNIT=<snap unit MN> / *AUTO-SELECT
Further sequence of operations
/UNLOCK-DISK is executed internally for all snap units (see the “Commands” manuals [1]) provided these are attached.
Console message NDE0050 indicates that the snap pair has been created and activated and has been assigned the COPY-ON-WRITE status. The status can also be displayed using /SHOW-SNAP-SESSION-STATUS.
/START-SNAP-SESSION enables the pubset to be optionally renamed.
The snap pubset can be imported. The associated MRSCAT entry is automatically created or updated.
Repeated entry of the /START-SNAP-SESSION command enables multiple snap sessions (multiple snap pairs) to be generated for an original unit.