HP StorageWorks XP Remote Web Console Software - Manual (Page 28)

28 TrueCopy for the XP128/XP1024/XP10000/XP12000
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 Volumes
When an MCU receives a read command for a TC390 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 TC390 R-VOL for recovery.
When an MCU receives a write command for a TC390 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 TC390A 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 TC390 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 TC390A
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 TC390A recordset for this
operation. If the update copy operation fails, the RCU suspends either the affected pair or all TC390A
pairs in the consistency group, depending on the type of failure. When the suspended TC390A pair or
group is resumed, the MCU and RCU negotiate the resynchronization of the pair(s). For more
information on suspended TC390A pairs, see ” Suspended TrueCopy Asynchronous Pairs ” on page 40.
The RCU does not allow a TC390 R-VOL to be online and rejects all host-requested read and write I/O
operations for a TC390 R-VOL. The TC390 R-VOLs must be offline during normal TC390 operations. TC390
provides a special R-VOL read option that allows read-only access to the R-VOL while the pair is suspended
(see ” R-VOL Read Option ” on page 28 for information on the R-VOL read option). If you need write access
to a TC390 R-VOL, you must delete the pair.
R-VOL Read Option
For additional flexibility, TC390 offers a special R-VOL read option. The HP representative enables the
R-VOL read option on the RCU (mode 20). You can use the TC390 R-VOL read option to read a TC390
R-VOL only while the pair is suspended, without having to delete the pair. The RCU will change only the
VOLSER of the suspended R-VOL so that the R-VOL can be online to the same host as the M-VOL while the
pair is suspended. All other write I/Os will be rejected by the RCU. The MCU copies the M-VOL VOLSER
back onto the R-VOL when the pair is resumed. When the R-VOL read option is not enabled and/or the
pair is not suspended, the RCU rejects all read and write I/Os to a TC390 R-VOL. If you need write access
to an R-VOL, you must delete the pair.
NOTE: For 2105 controller emulation, the CSUSPEND command to the R-VOL of a suspended TC390
pair will be rejected when the TC390 R-VOL read option is used.
Difference Management
The differential data (updated by write I/Os during split or suspension) between M-VOL and R-VOL is
stored in each cylinder bitmap. When a deleted/suspended pair is resumed (Resume Pair), the MCU
merges the M-VOL and R-VOL bitmaps, and the differential data is copied to the R-VOL. The unit of data