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 and Symmetrix this means that they must be in the UNUSED status from the point of view of snap unit usage. The definitions for snap unit usage in section "Snap units (VMAX3)" apply for VMAX3.
The snap pairs can be created during normal processing of the original unit. Snap pairs on ETERNUS DX/AF and VMAX3 are activated immediately when a command is executed. Snap pairs on a Symmetrix must be activated retroactively (see the next section).
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
ETERNUS DX/AF and VMAX3
/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.
/ACTIVATE-SNAP enables the pubset to be optionally renamed if this has not already been done with /START-SNAP-SESSION.
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.
Symmetrix
Console message NDE0050 indicates that the snap pair has been created and has assumed the CREATED status. The snap pair can then be activated using /ACTIVATE-SNAP.
Repeated entry of the /START-SNAP-SESSION command enables multiple snap sessions (multiple snap pairs) to be generated for an original unit. For Symmetrix this is possible only if no earlier snap or clone session in the RESTORED status exists. If such a snap or clone session in the RESTORED status does exist, it must be terminated before a new snap session is generated (/STOP-SNAP-SESSION or /STOP-CLONE-SESSION).