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

About TrueCopy for z/OS Operations 2-17
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Read and Write I/O Operations for TrueCopy for z/OS Volumes
When an MCU receives a read command for a TrueCopy for z/OS M-VOL, the
MCU completes the read from either cache or the M-VOL. If the read fails, the
redundancy provided by RAID technology recovers the failure. The MCU does
not read the TCz R-VOL for recovery.
When an MCU receives a write command for a TCz Synchronous M-VOL with
pending duplex status (and the track has already been copied to the R-VOL),
the MCU performs a synchronous update copy operation to complete the write
at the R-VOL. When an MCU receives a write command for a pending duplex
TCzA M-VOL (and the track has already been copied to the R-VOL), the MCU
performs an asynchronous update copy operation.
When an MCU receives a write command for an M-VOL with duplex status, the
user-selected update copy mode of the pair (synchronous or asynchronous)
determines the sequence of events:
• Synchronous Mode: The MCU performs the write operation on the M-VOL,
reports channel-end status to the host, starts the update copy operation for
the R-VOL, and then reports device-end status to the host only after the
update copy operation is complete. If the M-VOL write or R-VOL update copy
operation fails, the MCU reports a unit check, and the host system and
application program will regard that write operation to the M-VOL as failed. If
a failure occurs at the M-VOL or the R-VOL, the corresponding volume of the
TCz pair will decommit the update to maintain exact synchronization of the
volumes.
• Asynchronous Mode: The MCU completes M-VOL write operations
independently of the associated update copy operations at the R-VOL. The
RCU manages the R-VOL updates according to the TCzA recordset
information and maintains time-based data consistency for the R-VOLs. If the
M-VOL write operation fails, the MCU reports a unit check and does not create
the TCzA recordset for this operation. If the update copy operation fails, the
RCU suspends either the affected pair or all TCzA pairs in the consistency
group, depending on the type of failure. When the suspended TCzA pair or
group is resynchronized, the MCU and RCU negotiate the resynchronization of
the pair(s).
The RCU does not allow a TCz R-VOL to be online and rejects all host-
requested read and write I/O operations for a TCz R-VOL. The TCz R-VOLs
must be offline during normal TCz operations.
Note: TrueCopy for z/OS provides a special R-VOL read option which allows
read-only access to the R-VOL while the pair is suspended. If you need write
access to a TCz R-VOL, you must release the pair.