HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

MCU. The RCU ensures that records are applied to the S-VOLs i n the correct sequence. This method of
remote I/O provides the most efficient use of MCU-to-RCU link resources.
Note: The p arameter leng th and detailed specification of the Continuous Access Asynchronous channel
command are different than for Continuous Access Synchronous RIOs. You must make su re that your
channel extenders are c apable of supporting this command. For further details, contact your HP
support representative.
Storing Recor
dsets
at the RCU
The RCU mainta
ins
queues to control the storing of recordsets in the sidefile a nd commitment of updating
records in th
e
S-VOLs. The RCU queuing mechanism for 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.
Note: TheMCU does notremovethe sidefile entry for a recordset from its cache until it re ceives an I/O
completion s
ignal (device end) from the RCU. This is true even if the MCU and RCU are connected via a
channel ext
ender 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 Se ttling Recordsets at the RCU
The RCU selects the recordset to b e 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 which
has
the lowest sequence number, a nd then settles that recordset.
3. The RCU repeats steps (1) and (2) to select and settle Continuous Access Asynchronous
recordsets.
Figure 3 on page 24 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 S 1 to be settled because S1 is the
lowest sequence number. When S 1 is removed from the MCU queue, recordset S2 b ecomes the top
entry, but it is empty. W hen recordset S2 arrives, the RCU selects S2 a s 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 a re 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.
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