Hitachi TrueCopy for z/OS user guide (December 2005)

34 TrueCopy for the XP128/XP1024/XP10000/XP12000
In the following figure, updates to the primary volume (A) are transferred to the device addresses of its
secondary volume (A’) as well as the secondary volumes of dummy volume pairs (B’-D’) in parallel.
Figure 6 Dummy pair mode
The basic specifications for dummy pair mode are:
• Up to three dummy pairs can be related to one real volume pair.
• Volumes must be assigned in both the MCU and RCU for dummy pair.
• Custom-size volumes of minimum size (one cylinder) can be used for dummy pair.
• Dummy pair volumes cannot accept host I/Os, must be offline from all host systems.
• Volumes for a dummy pair must be in the same logical CU image as the real pair.
The following table shows the main differences between dummy pair mode and HPAV operations.
General recommendation: When a specific TC390A volume could be a bottleneck due to high write I/O
activity:
• If write I/Os are from independent I/O requester, consider dividing I/O activity to different volume (by
dataset relocation, for example).
• Otherwise consider using TC390A dummy pair(s).
You can use the PPRC CESTPAIR command as follows to establish TC390A dummy pairs and relationships
to real volume pairs. For detailed information on configuring master and dummy pairs using the CESTPAIR
TSO command, see ”CESTPAIR” on page 152.
1. CESTPAIR: Establish real volume pair (A-A’)
2. CESTPAIR: Establish 1st dummy pair (B-B’) for A-A’
3. CESTPAIR: Establish 2nd dummy pair (C-C’) for A-A’
4. CESTPAIR: Establish 3rd dummy pair (D-D’) for A-A’
Table 8 Differences between TC390A dummy pair mode and HPAV
TC390A Dummy Pair HPAV
Paralleling TC390 Async RIO Host I/O
Effective for Single or independent I/O requester Independent I/O requester (for example, Different job
and extend). Due to this reason, possibly HPAV is not
effective for IMS® WADS.
Requires At least minimum capacity VSC custom
volume
Only LDEV address