This topic addresses
the physical and operational considerations
for PPRC. Topics include conditions for establishing PPRC paths and
volume format restrictions.
The following considerations apply
to PPRC operations:
- Fixed Block (Open) devices in PPRC sessions can be controlled
by TSO and ANTRQST commands as long as there is a CKD device configured
in the same cluster as the FB devices.
- To
ensure data integrity,
all CKD volumes that are part of PPRC copy operations must
conform to the following volume format, track, and access method restrictions:
- Volumes must have a standard format for record zero (R0).
Volumes
with R0 data lengths longer than 8 bytes can cause a track format
error to remain undetected when the storage control formats the track
in cache.
- You cannot assign alternate tracks in the user area.
User data
can be overlaid if you assign a user track on the primary address
as an alternate track for a secondary address.
- All storage
control extent specification commands must
specify normal access authorization mode. Data written to a PPRC primary
device while the storage control is in diagnostic or device support
mode is not copied to the PPRC secondary volume. It is therefore important
to remove volumes from PPRC pairs before running a utility
program like ICKDSF.
- PPRC secondary volumes can
only accept a subset of I/O operations
while they are secondary devices (much like dual copy volumes). IBM® recommends that you vary the
attached secondary volumes offline to all attached systems. The PPRC
secondary volume serial numbers are the same as those on the primary
volumes.
- PPRC allows the mixed use of ESCON® and
FCP connections.
- ESCON connections
are
single directional. You cannot use an adapter as both the source of
one path and the destination of another path. But you can establish
multiple paths using the adapter as a source, or multiple paths using
the adapter as a destination.
- The FCP physical connection
can be used in both directions, and
can have many logical paths established on that connection, some going
in opposite directions.
The following table summarizes
some of the differences between PPRC FCP and PPRC ESCON technology:
| FCP
compared to ESCON |
FCP |
ESCON |
|---|
| PPRC primary port accepts host I/O |
Yes |
No |
| PPRC secondary
port accepts host I/O |
Yes |
Yes |
| Link is full-duplex (have paths established
in both directions simultaneously) |
Yes |
No |
| Supports synchronous PPRC |
Yes |
Yes |
| Supports
PPRC Extended Distance |
Yes |
Yes |
| Supports Global Mirror |
Yes |
No |
| Number of PPRC logical paths defined
between
the primary and secondary LSSs |
8 |
8 |
| Number of logical paths from a primary
ESS (32
PPRC paths between one primary and four secondary ESSs) |
1024 |
1024 |
- For
VM Systems: Native VM supports PPRC
with VM/ESA 2.1.0 and above.
Observe the following when you operate PPRC as a guest under VM/ESA 2.1.0 and above:
- PPRC volumes must either be dedicated volumes, or you must
define
them as fullpack minidisks. This includes DEVNO-defined minidisks.
- The VM guest directory must include an entry that states "STDEVOPT
DATAMOVER=YES".
For VM/ESA levels
prior
to 2.1.0, you must define PPRC volumes to VM as unsupported disk,
and you cannot define them as minidisks.