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

are applied to the S-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 the XP Continuous Access Asynchronous channel
command are different than for XP Continuous Access Synchronous RIOs. Ensure 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 S-VOLs. The RCU queuing mechanism for XP Continuous Access
Asynchronous uses the sequence numbers provided by the MCU SVP to control the sequence in
which S-VOL updates are applied and to check for 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
over 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 (that is, the recordset has not yet been received), the RCU waits for
that entry.
2. When the top of each queue contains a valid entry (recordset), the RCU selects the entry that
has the lowest sequence number, and then settles that recordset.
3. The RCU repeats steps 1 and 2 to select and settle XP Continuous Access Asynchronous
recordsets.
Figure 3 (page 23) illustrates recordset selection and settling at the RCU. In this example, the top
of the queue contains a valid entry: S1. The RCU selects recordset S1 to be settled because S1 is
the lowest sequence number. When S1 is removed from the MCU queue, recordset S2 becomes
the top entry, but it is empty. When recordset S2 arrives, the RCU selects S2 as the next recordset
to be settled. The recordset selected by the RCU is marked as host-dirty and treated as formal data.
The RCU settles the updated records in the recordset as follows:
•
If the corresponding track is in cache (track-hit), the updated records in the recordset are
copied to the existing cached track, and the cache space for the sidefile is released.
•
If the corresponding track is not in cache (track-miss), the RCU changes the cache designation
of the sidefile to formal data. The data is not physically moved.
22 About XP Continuous Access Operations