Creating And Storing Recordsets At The Mcu; Sending Recordsets To The Rcu; Storing Recordsets At The Rcu; Selecting And Settling Recordsets At The Rcu - HP StorageWorks XP12000 User Manual

Creating and storing recordsets at the MCU

When an MCU performs an update (host-requested write I/O) on a P-VOL, the MCU creates a recordset
containing the updated record, sequence number, record location (device, cylinder, track, and record
number), and record length. Recordsets are queued in cache storage of the MCU and sent to the RCU
independent of host I/O processes. The RCU utilizes sequence number information in the recordsets
to update S-VOLs in the same order as P-VOLs.
The sequence number indicates the number of recordsets the MCU created for each consistency group.
Recordset information, except for updated records, is stored and queued in an area of cache known as
sidefile cache (see
NOTE:
Continuous Access XP Asynchronous operations continue uninterrupted if the disk array reboots or the
disk array's internal Service Processor (SVP) fails.

Sending recordsets to the RCU

The MCU sends asynchronous Continuous Access XP recordsets to the RCU in a manner similar to
Continuous Access XP 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 recordsets in
FBA format (not CKD) using a single channel command, eliminating overhead associated with FBA-CKD
conversion and thus providing more efficient data transfer. The MCU can send several recordsets using a
single RIO, even if their sequence numbers are not contiguous. Therefore, 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 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 Continuous Access XP Asynchronous channel
command are different than for Continuous Access XP Synchronous RIOs. You must ensure that
your channel extenders support this command. For further details, contact your HP account support
representative.

Storing recordsets at the RCU

The RCU maintains queues to control storing recordsets in the sidefile and committing updated records
in the S-VOLs. The RCU queuing mechanism for Continuous Access XP Async uses sequence numbers
provided by the MCU 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 via 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 recordsets 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 (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 with the
earliest sequence number, and settles that recordset.
3.
The RCU repeats this process to select and settle Continuous Access XP Asynchronous recordsets.
Figure 3 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 earliest
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
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Continuous Access XP for the Array
Inflow control of recordsets).