Snapshots are generated in VMAX3 as internal "Point-in-Time copies." They initially have no logical unit number (LUN). A server cannot access them directly.
Snapshots are managed in VMAX3 using the unique snapshot name.
Random, unused volumes are used to access snapshots. These volumes are connected with the snapshot. The connection of snapshot and snap unit is dynamic, i.e. a snap unit can by turns be assigned to different snapshots. The connection can be canceled without terminating the snapshot. Up to 1024 volumes can also be connected with the snapshot.
Storage areas for the data of the snapshot are allocated from the same thin pool as for the original unit.
In SHC-OSD snapshots in the /START-SNAP-SESSION command are always connected with a snap unit.
Snapshots without a connection to a snap unit can be managed using the VMAX3's management software. In SHC-OSD itself, no snapshots can currently be generated without a connection to a snap unit.
No preconfigured volumes and no save pool are required for TimeFinder SnapVX.
Snapshot name
The snapshot name is assigned for a voöume or pubset in the /START-SNAP-SESSION command, and designates the snapshot for all further actions.
The snapshot name is an alphanumeric string with at least one and at most 32 characters. The special characters "-" (hyphen) and "_" (underscore) can be specified. A distinction is made between uppercase and lowercase.
When SNAPSHOT-NAME = *BY-RULE is specified, SHC-OSD forms the snapshot name according to the rule: SHC-OSD-<mn1>-<mn2> (e.g. SHC-OSD-4740-4762), the following applying here:
When a unit is specified,
<mn1>is the mnemonic or logical unit number of the original unit, and<mn2>is the mnemonic or logical unit number of the snap unit.When a pubset is specified,
<mn1>is the mnemonic or pubres of the original unit, and<mn2>is the mnemonic of the snap unit for the pubres.
The snapshot name is output with the /SHOW-SNAP-SESSION command.
CAUTION!
No check is made to see whether a snapshot name applies for one or for more than one volume. Snapshots can be created for further volumes with the same name. The resulting snapshot is then possibly not consistent. The consistency must be ensured by the caller.
Snapshot generations
When further snapshots with the same name are generated for a volume or pubset, TimeFinder SnapVX manages these snapshots as new generations for the existing snapshot. Up to 255 generations are possible.
Generations can be created using the VMAX3's management software.
Generations are displayed in the /SHOW-SNAP-SESSION command.
In SHC-OSD itself no generations can currently be displayed; generation 0 is always worked with.
Generation 0 is always created when a snapshots is created. If a snapshot with this name already exists, its existing generations are incremented by 1 (the previous generation 0 becomes generation 1, generation 1 becomes generation 2, etc.). The generations are sorted chronologically in the order in which they were created.
The latest generation (0) is always worked with. When a snapshot is terminated, generation 0 s terminated. If the terminated snapshor has further generations, its generations are decremented by 1 (the previous generation 1 becomes generation 0, generation 2 becomes generation 1, etc.).
Individual generations of a snapshot can be addressed directly via the VMAX3's management software (exception: creation of the snapshot).
Automatic selection of snap units
VMAX3 knows no special snap volumes. Basically all configured volumes can be used as snap units.
The SNAP-UNIT = *AUTO-SELECT operand in the /START-SNAP-SESSION command is used for automatic selection of suitable BS2000 volumes as snap units.
This is important in particular when using the "Snapsets" function (see the “Introduction to System Administration” manual [5]) .
When BS2000 volumes are to be used automatically as snap units, they must be initialized in advance with the VOLIN utility (see the “Utility Routines” manual [8]).
The special notation S#<mn> is introduced as the VSN for these volumes, e.g. S#5234, where <mn> is the device mnemonic.
Such volumes prepared especially in BS2000 for the *AUTO-SELECT option are not displayed with the UNUSED status in the /SHOW-SNAP-SESSION-STATUS command. Selection in the /SHOW-SYMMETRIX-DEVICE-CONFIG command also has no effect for these volumes.
When the snap session is terminated with /STOP-SNAP-SESSION, the special VSN S#<mn> can be discarded or reconstructed with the SNAP-UNIT=*DESTROY/*UNUSED operand. When the special VSN is reconstructed, the volume is still included in the automatic selection.
Limiting the number of snap units
Currently a maximum of 127 snap units can be configured for an original unit in VMAX3 .
In SHC-OSD the use of 127 snap units is preset. This setting cannot be changed for TimeFinder SnapVX (/MODIFY-SHC-PROCESSING).