HP StorageWorks XP Remote Web Console Software - Manual (Page 19)

Figure 3 Selecting and settling XP Continuous Access 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 XP Continuous
Access 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
array uses certain parameters for XP Continuous Access Asynchronous cache inflow control. Use
the XP Continuous Access Async Option pane to modify the following parameters (see
“Asynchronous copy option” (page 86) ):
•
Pending Update Data Rate (%): Sidefile threshold = maximum cache % available for use by
XP Continuous Access Asynchronous sidefiles.
•
Offloading Timer (sec.): Maximum time between XP Continuous Access 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 (pending update
data rate) (I/O delay start), the array's I/O response is delayed. When the amount of MCU sidefile
cache reaches the user-specified threshold (I/O delay increase), the array's I/O delay starts
increasing. If the MCU cannot send a recordset to the RCU within the user-specified offloading
timer value because the amount of MCU sidefile cache exceeded the sidefile threshold continuously,
the MCU suspends all affected XP Continuous Access Asynchronous volume pairs and resets the
State Change Pending (SCP) condition to avoid hanging up the system.
Inflow control by RCU
When the amount of RCU sidefile cache reaches the user-specified threshold, the RCU responds
with channel-command-retry requests to the RIO commands that transfer recordsets from the MCU.
XP Continuous Access Asynchronous recordset operations 19