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

2-16 About TrueCopy for z/OS Operations
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Update Copy Operation
A TrueCopy for z/OS update copy operation occurs when the host issues a
write I/O operation to the M-VOL of a TCz pair. The update copy operation
duplicates the M-VOL write I/O at the R-VOL to keep the M-VOL and R-VOL
synchronized. TCz provides two modes for update copy operations:
synchronous and asynchronous. The update copy mode is specified when you
add a TCz pair and cannot be changed. TCz also allows you to specify whether
the cache-fast-write (CFW) data is included in the update copy operations.
For synchronous update copy mode, the MCU ensures that the M-VOL and R-
VOL are synchronized at all times. The MCU does not return device-end status
for the M-VOL write I/O until both the M-VOL write and its associated update
copy operation at the RCU are complete. For synchronous mode, the MCU
starts the update copy operation when it receives:
• the last write command in the current domain, specified by the preceding
locate record command, or
• a write command which requires switching to the next track, or
• a write command which was not preceded by a locate record command.
Note: If many consecutive records are updated by a single CCW chain which
does not use the locate record command (e.g., long sequential chained write
operations), storage system performance may be significantly impacted.
For asynchronous update copy mode, the MCU stores the M-VOL updates
along with additional control information in cache, and sends the updates and
control information to the RCU completely independent of the host I/O
processes. These updates with their associated control information are called
TCzA recordsets. The RCU stores the TCzA recordsets in cache and performs
the updates to the R-VOLs in the same order as they were performed at the
MCU(s) according to the TCzA time-stamp and sequence information.
Priority of initial and update copy: In both TrueCopy for z/OS Sync and
Async, update copy has higher priority than initial copy. However, initial copy
is executed based on the copy pace (3 or 15 tracks); therefore, update copy
must wait this interval if initial copy is being executed. For example, if the
copy pace is 15 tracks, the update copy may wait up to 15 tracks (1 cylinder).
In the case of TrueCopy for z/OS Async, update copy is executed
asynchronously, but the same scheduling conflict can occur between the
asynchronous update copy (write recordset) and initial copy.