HP StorageWorks XP Remote Web Console Software - Manual (Page 21)

To calculate the maximum number of pairs that you can create:
The maximum number of pairs that you can create =
(
↓ (the total number of bitmap areas in the
storage system ÷ Number of bitmap areas) )
The … symbols around a value indicate that the value should be rounded down to the former
integer.
CAUTION: The bitmap areas that are used for XP Continuous Access are shared with Hitachi
TrueCopy™ for Mainframe, XP Continuous Access Journal, Hitachi Universal Replicator™ for
Mainframe, and XP External Storage Access Manager Software. If you use XP Continuous
Access with TrueCopy for Mainframe, XP Continuous Access Journal, Universal Replicator for
Mainframe, and XP External Storage Access Manager Software, then use the total number of
each set of pairs to calculate the maximum number of pairs. Also, if XP Continuous Access
and XP Continuous Access Journal share the same volume, use the total number of both pairs,
whether the shared volume is primary or secondary.
XP Continuous Access Asynchronous Recordset Operations
The XP Continuous Access Asynchronous recordsets contain the P-VOL updates and the associated
control information, including the sequence number of the P-VOL update, which enables the RCU
to maintain update consistency of the XP Continuous Access Asynchronous S-VOLs. XP Continuous
Access Asynchronous recordset operations include:
•
“Creating and Storing Recordsets at the MCU” (page 21)
•
“Sending Recordsets to the RCU” (page 21)
•
“Storing Recordsets at the RCU” (page 22)
•
“Selecting and Settling Recordsets at the RCU” (page 22)
•
“Types of Recordsets” (page 23)
•
“Inflow Control of Recordsets” (page 23)
Creating and Storing Recordsets at the MCU
When an MCU performs an update (host-requested write I/O) on an XP Continuous Access P-VOL,
the MCU creates an XP Continuous Access Asynchronous recordset that contains: sequence number,
record location (device, cylinder, track, record number), and record length. The XP Continuous
Access Asynchronous recordsets are queued in the cache storage of the MCU and sent to the RCU
independent of host I/O processes. The RCU utilizes the sequence number information in the
recordsets to update the S-VOL(s) in the same order as the P-VOL(s).
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.
XP Continuous Access Asynchronous operations continue uninterrupted if the SVP reboots or even
if the SVP fails.
Sending Recordsets to the RCU
The MCU sends the XP Continuous Access Asynchronous recordsets to the RCU in a manner similar
to how the XP Continuous Access Synchronous updates are sent. 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 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 data. The MCU can send several recordsets using a single RIO, even if their sequence
numbers are not contiguous. Therefore, XP Continuous Access Asynchronous recordsets are usually
sent to the RCU in a different order than are the arrivals at the MCU. The RCU ensures that records
XP Continuous Access Asynchronous Recordset Operations 21