Read and write i/o operations for truecopy volumes – HP StorageWorks XP Remote Web Console Software User Manual
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Hitachi TrueCopy z/OS for HP XP12000/XP10000 and SVS 200 storage systems
its associated update copy operation at the RCU are complete. For synchronous mode, the MCU starts the
update copy operation when it receives one of the following:
•
The last write command in the current domain, specified by the preceding locate record command.
•
A write command that requires switching to the next track.
•
A write command that was not preceded by a locate record command.
NOTE:
If many consecutive records are updated by a single CCW chain that does not use the locate
record command (for example, long sequential chained write operations), disk array 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 TC390A
recordsets. The RCU stores the TC390A 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 TC390A time-stamp and sequence
information. For more information on TC390A recordset operations, see ”
Priority of initial and update copy: In both TC390 Synchronous and TC390A, 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 TC390A, 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 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
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
” on page 27 for information on the R-VOL read option). If you need write access
to a TC390 R-VOL, you must delete the pair.