P/DAS commands allow the system operator to redirect application I/Os that are currently sent to the PPRC primary volume to instead go to the secondary volume. PPRC volume pairs must be in duplex state for the P/DAS function to perform.
P/DAS manages all I/O redirection at the I/O supervisor level. P/DAS operations are therefore nearly transparent to any application program that reads from or writes to the volume. Based on the options you select, the redirection of the I/O can continue across system IPLs. The options can apply to all systems in the sysplex that are attached to the volumes.
The P/DAS function performs device switching and copying functions, as does dual copy. If you are familiar with dual copy operation, a comparison of both functions might be helpful. Table 1 summarizes some of the operating differences between the dual copy and P/DAS functions.
| Function | P/DAS | Dual copy |
|---|---|---|
| How is switching accomplished? | Nontransparently | Transparently |
| Where does the operation occur? | Between storage controls | Within the same storage control |
| How is the function controlled? | By the operator | By the subsystem |
| Which system component manages the swap? | MVS system | Storage subsystem |
| How is I/O addressed? | Operator specifies target address | Operator specifies source address |
P/DAS operates similarly to DDR Swap. After a swap operation, the "To" device comes online and the "From" device goes offline.