HP XP Continuous Access Software - Manual (Page 21)

Continuous Access XP user guide for the XP1024/XP128 21
to the MCU. When you resync a split Continuous Access XP pair that has the S-VOL write option enabled,
the RCU sends the S-VOL cylinder bitmap to the MCU, and the MCU merges the P-VOL and S-VOL
bitmaps to determine which cylinders are out-of sync. This ensures proper resynchronization of the pair.
Continuous Access XP Asynchronous recordset operations
Continuous Access XP Asynchronous recordsets contain P-VOL updates and associated control
information, including sequence number of the P-VOL update, which enables the RCU to maintain update
consistency of S-VOLs. Continuous Access XP Recordset operations include:
• ” Creating and storing recordsets at the MCU ” on page 21
• ” Sending recordsets to the RCU ” on page 21
• ” Storing recordsets at the RCU ” on page 21
• ” Selecting and settling recordsets at the RCU ” on page 22
• ” Types of recordsets ” on page 22
• ” Inflow control of recordsets ” on page 22
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 ” Inflow control of recordsets ” on page 22).
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.