HP StorageWorks XP Remote Web Console Software - Manual (Page 20)

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.
If many consecutive records are updated by a single CCW chain which does not use the locate
record command (for example, 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 TrueCopy for Mainframe Asynchronous recordsets. The RCU stores the TrueCopy for
Mainframe Asynchronous 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 TrueCopy for Mainframe Asynchronous
time-stamp and sequence information.
Priority of initial and update copy: In both TrueCopy for Mainframe 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 Mainframe Async, update copy is executed asynchronously, but the same
scheduling conflict can occur between the asynchronous update copy (write recordset) and initial
copy.
Read and Write I/O Operations for TrueCopy for Mainframe Volumes
When an MCU receives a read command for a TrueCopy for Mainframe 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 TrueCopy for Mainframe R-VOL
for recovery.
When an MCU receives a write command for a TrueCopy for Mainframe 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 TrueCopy for Mainframe Asynchronous 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 TrueCopy for Mainframe 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 TrueCopy for Mainframe Asynchronous 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 TrueCopy for Mainframe Asynchronous recordset for this
20 About TrueCopy for Mainframe Operations