Hitachi TrueCopy for IBM z/OS User and Reference Guide (T5211-96001, June 2007)

2-20 About TrueCopy for z/OS Operations
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Sending Recordsets to the RCU
The MCU sends the TCzA recordsets to the RCU in a similar manner to the TCz
Synchronous updates. The MCU’s initiator ports act as host processor channels
and issue special I/O operations, called remote I/Os (RIOs), to the RCU. The
RIO transfers the recordsets in FBA format (not CKD) using a single channel
command, eliminating the overhead associated with FBA-CKD conversion and
thus providing more efficient transfer of user data. The MCU can send several
recordsets using a single RIO, even if their sequence numbers are not
contiguous. Therefore, TCzA recordsets are usually sent to the RCU in a
different order than the arrivals at the MCU. The RCU ensures that records are
applied to the R-VOLs in the correct sequence. This method of remote I/O
provides the most efficient use of MCU-to-RCU link resources.
Note: The parameter length and detailed specification of this TCzA channel
command are different than for TCz Synchronous RIOs. You must make sure
that your channel extenders are capable of supporting this command. For
further details, please contact your Hitachi account team.
Storing Recordsets at the RCU
The RCU maintains queues to control the storing of recordsets in the sidefile
and commitment of updating records in the R-VOLs. The RCU queuing
mechanism uses time-stamping to control the sequence in which R-VOL
updates are applied, and uses sequence numbers provided by the MCU to
check for any missing updates.
Note: The MCU does not remove the sidefile entry for a recordset from its
cache until it receives an I/O completion signal (device end) from the RCU.
This is true even if the MCU and RCU are connected via a channel extender
product. If a recordset is lost in transmission from the MCU to the RCU, the
MCU’s differential data ensures that the missing recordset is identified and
resent to the RCU.