HP XP Continuous Access Software - Manual (Page 23)

Continuous Access XP user guide for the XP1024/XP128 23
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
threshold (I/O delay increase), the disk array’s I/O delay starts increasing. If the MCU cannot send a
recordset to the RCU within the user-specified offloading timer value, the MCU suspends all affected
Continuous Access XP Async volume pairs and resets the 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. The RCU
accepts only the recordset with the sequence number required to continue settling pending recordsets. If
the RCU cannot settle a recordset within the user-specified offloading timer value, the RCU suspends all
Continuous Access XP Asynchronous volume pairs and resets the channel-command-retry condition to
avoid hanging up the MCU.
Table 4 shows sidefile threshold values for Continuous Access XP Asynchronous operations and write
pending operations and describes actions occurring when each threshold is reached.
HP Cache LUN XP operations might decrease total amount of cache available for Continuous Access XP
Asynchronous operations, but they do not directly affect sidefile cache usage. Available cache is the
amount of physical cache memory installed on the disk array minus any cache reserved for the HP Cache
LUN XP feature.
If you do not want to control inflow into a sidefile, specify the value of the Pending Update Data Rate for
the I/O Delay Start and I/O Delay Increase, and specify 0 seconds for the Offloading Timer.
Continuous Access XP Asynchronous consistency group operations
Continuous Access XP Asynchronous consistency groups maintain update sequence consistency across an
aggregated data set spanning a group of volumes. P-VOLs and S-VOLs of the pairs in a consistency group
must be located within one physical MCU and one physical RCU (1-to-1 requirement). Continuous Access
XP consistency group operations include:
• Group options
• Group consistency time
• Group operations
Table 4 Sidefile threshold values for Continuous Access XP Asynchronous operations and write pending
operations
Operation Thresholds Actions
Continuous Access
XP Asynchronous
I/O Delay Start (threshold) = 30%
Use
Continuous Access XP to adjust threshold to
desired value between 0 and 70% in 1%
increments. Specify a value not exceeding the
value of I/O Delay Increase, and Sidefile threshold.
MCU reaches threshold: I/O delay
starts. If sidefile threshold is set
between 0 and 30%, influence to
the I/O delay is kept low.
RCU reaches threshold: No action.
Continuous Access
XP Asynchronous
I/O Delay Increase (threshold) = 40%
Use
Continuous Access XP to adjust threshold to
desired value between 0 and 70% in 1%
increments. Specify a value within the range of the
I/O Delay Start threshold and Sidefile threshold.
MCU reaches threshold: I/O delay
increases. If sidefile threshold is set
between 0 and 30%, influence to
the I/O delay is kept low.
RCU reaches threshold: No action.
Continuous Access
XP Asynchronous
Sidefile threshold = 50%
Use
Continuous Access XP to adjust threshold
(30, 40, 50, 60, or 70%).
MCU reaches threshold: I/O delay.
RCU reaches threshold: Command
retry to MCU.
Write Pending [write pending] / [avail cache - sidefile] = 70% Command retry.