HP StorageWorks XP Remote Web Console Software - Manual (Page 30)

30 TrueCopy for the XP128/XP1024/XP10000/XP12000
• Sending recordsets to the RCU
• Storing recordsets at the RCU
• Selecting and settling recordsets at the RCU
• Types of recordsets
• Inflow control for sidefiles
• Dummy pair mode
Creating and Storing Recordsets at the MCU
When an MCU performs an update (host-requested write I/O) on a TC390A M-VOL, the MCU creates a
TC390A recordset that contains: the updated record, time-stamp information, sequence number, record
location (device, cylinder, track, record number), and record length. The TC390A recordsets are queued in
the cache storage of the MCU and sent to the RCU independent of host I/O processes. The RCU uses 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. For more information on sidefile cache, see ” Inflow Control of
Recordsets ” on page 32.
Sending Recordsets to the RCU
The MCU sends the TC390A recordsets to the RCU in a similar manner to the TC390 Synchronous
updates. The MCU’s RCPs and/or 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, TC390A 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.
NOTE: The parameter length and detailed specification of this TC390A channel command are different
than for TC390 Synchronous RIOs. Verify that your channel extenders are capable of supporting this
command. For further details, contact your HP account support 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.
NOTE: 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 through
a channel extender product. If a recordset is lost in transmission from the MCU to the RCU, the MCU’s
cylinder bitmap 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.