Recovering a shared control data set

You should always run with at least two active and one spare SHCDSs. If a permanent I/O error occurs for an active SHCDS, or if an SHCDS becomes inaccessible from one or more systems, it is automatically replaced by one of the spare SHCDSs. When a system is forced to run with only one SHCDS, it issues a message requesting that you add another active SHCDS and at least one spare SHCDS. If any system does not have access to an SHCDS, all opens for VSAM RLS processing are prevented on that system until an SHCDS becomes available.

The information in SHCDSs is continuously updated. Therefore, backup/restore procedures for SHCDSs are ineffective.

Use the following command formats for the VARY SMS command to add and delete SHCDSs:

To add a new, active SHCDS:
```
VARY SMS,SHCDS(SHCDS_name),NEW
```
To add a new, spare SHCDS:
```
VARY SMS,SHCDS(SHCDS_name),NEWSPARE
```
To delete either an active or a spare SHCDS:
```
VARY SMS,SHCDS(SHCDS_name),DELETE
```