HP StorageWorks XP Remote Web Console Software - Manual (Page 22)

Creating and Storing Recordsets at the MCU
When an MCU performs an update (host-requested write I/O) on a TrueCopy for Mainframe
Asynchronous M-VOL, the MCU creates a TrueCopy for Mainframe Asynchronous recordset which
contains: the updated record, time-stamp information, sequence number, record location (device,
cylinder, track, record number), and record length. The TrueCopy for Mainframe Asynchronous
recordsets are queued in the cache storage of the MCU and sent to the RCU independent of host
I/O processes. The RCU utilizes the time-stamp and sequence number information in the recordsets
to update the R-VOL(s) in the same order as the M-VOL(s).
The time-stamp information is acquired from the (MVS) host's I/O time-stamp function. This time
stamp provides a protective measure for write-dependent applications and minimizes recovery
time in the event of a disaster. The sequence number indicates the number of recordsets that the
MCU has created for each consistency group. The recordset information, except for the updated
records, is stored and queued in an area of cache known as sidefile cache.
Sending Recordsets to the RCU
The MCU sends the TrueCopy for Mainframe Asynchronous recordsets to the RCU in a similar
manner to the TrueCopy for Mainframe Synchronous updates. The MCU's initiator ports act as
host processor channels and issue special I/O operations, called remote I/Os (RIOs), to the RCU.
The RIO transfers the recordsets in FBA format (not CKD) using a single channel command,
eliminating the overhead associated with FBA-CKD conversion and thus providing more efficient
transfer of user data. The MCU can send several recordsets using a single RIO, even if their
sequence numbers are not contiguous. Therefore, TrueCopy for Mainframe Asynchronous recordsets
are usually sent to the RCU in a different order than the arrivals at the MCU. The RCU ensures that
records are applied to the R-VOLs in the correct sequence. This method of remote I/O provides
the most efficient use of MCU-to-RCU link resources.
The parameter length and detailed specification of this TrueCopy for Mainframe Asynchronous
channel command are different than for TrueCopy for Mainframe Synchronous RIOs. You must
make sure that your channel extenders are capable of supporting this command. For further details,
contact your HP service representative.
Storing Recordsets at the RCU
The RCU maintains queues to control the storing of recordsets in the sidefile and commitment of
updating records in the R-VOLs. The RCU queuing mechanism uses time-stamping to control the
sequence in which R-VOL updates are applied, and uses sequence numbers provided by the MCU
to check for any missing updates.
The MCU does not remove the sidefile entry for a recordset from its cache until it receives an I/O
completion signal (device end) from the RCU. This is true even if the MCU and RCU are connected
via a channel extender product. If a recordset is lost in transmission from the MCU to the RCU, the
MCU's differential data ensures that the missing recordset is identified and resent to the RCU.
Selecting and Settling Recordsets at the RCU
The RCU selects the recordset to be promoted to formal data (or settled) as follows:
1. The RCU checks for a valid entry at the top of each queue in the consistency group. If the top
of any queue is empty (For example, recordset not yet received), the RCU waits for that entry.
2. When the top of each queue contains a valid entry (recordset), the RCU selects the entry which
has the earliest time-stamp value, and then settles this recordset.
3. The RCU repeats steps (1) and (2) to select and settle TrueCopy for Mainframe Asynchronous
recordsets.
“Selecting and Settling TrueCopy for Mainframe Async Recordsets at the RCU” (page 23) illustrates
recordset selection and settling at the RCU. In this example, the top of the queue contains a valid
entry: S1/T1. The RCU selects recordset S1/T1 to be settled, because T1 is the earliest time-stamp.
22 About TrueCopy for Mainframe Operations