Selecting And Settling Recordsets At The Rcu; Types Of Recordsets; Inflow Control Of Recordsets; Selecting And Settling Continuous Access Xp Async Recordsets At The Rcu - HP xp12000 User Manual

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 recordset the RCU
selects 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), updated records in the recordset are copied to the
existing cached track, and 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. Data is not physically moved.
Figure 3

Selecting and settling Continuous Access XP Async recordsets at the RCU

Types of recordsets

In addition to host update recordsets, the MCU passes control information to the RCU in special
non-update recordsets. These special recordsets indicate when volume pair status changes, indicate when
an MCU power-off sequence is initiated, and maintain sequence numbers in periods of low host activities.

Inflow control of recordsets

As described in previous sections, both the MCU and RCU create sidefiles for storing Continuous Access
XP Asynchronous recordsets. Because sidefiles occupy exclusive space in cache, both the MCU and RCU
perform inflow control to prevent an overload of the disk array's cache resources. The XP12000/XP10000
uses certain parameters for Continuous Access XP Asynchronous cache inflow control. Use the Continuous
Access XP Async Option pane to modify the following parameters (see
page 82):
• Pending Update Data Rate (%): Sidefile (Continuous Access XP) threshold = maximum cache %
available for use by Continuous Access XP Asynchronous sidefiles.
• Offloading Timer (sec.): Maximum time between Continuous Access XP Async recordset transfers.
• I/O Delay Start (%): Maximum amount of cache for starting I/O inflow control.
• I/O Delay Increase (%): Maximum amount of cache for starting to strengthen control over I/O inflow.
Inflow control by MCU
When the amount of MCU sidefile cache reaches the user-specified threshold (I/O delay start), the disk
array's I/O response is delayed. When the amount of MCU sidefile cache reaches the user-specified
22 Continuous Access XP for the XP12000/XP10000
"Asynchronous copy option" on