Hitachi TrueCopy for IBM z/OS User and Reference Guide (T5211-96001, June 2007)

About TrueCopy for z/OS Operations 2-23
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Inflow Control of Recordsets
As described in the previous sections, both the MCU and RCU create sidefiles
for storing TCzA recordsets. Since the sidefiles occupy exclusive space in
cache, both the MCU and RCU perform inflow control to prevent an overload of
the storage system’s cache resources. The USP V uses the following
parameters for TCzA cache inflow control:
• Sidefile Threshold = maximum cache % available for use by TCzA sidefiles.
Table 2-4 shows the TCzA sidefile threshold values and describes the actions
that occur when each threshold is reached.
– The “high-water mark” (HWM), which is the sidefile threshold minus 20%,
indicates when graduated delay begins for TCzA pairs (see next section).
If system option mode 118 is ON, a warning SIM is also generated.
– The puncture threshold, which is the sidefile threshold plus 10%, indicates
when the TCzA pairs will be suspended due to sidefile overflow.
– Inflow control by MCU: When the amount of MCU sidefile cache
reaches the HWM, the MCU responds to update I/Os from the host with
state-change-pending (SCP) or channel-command-retry requests and
begins graduated delay.
– Inflow control by RCU: When the amount of RCU sidefile cache reaches
the HWM, the RCU responds with channel-command-retry requests to the
RIO commands which transfer the recordsets from the MCU. The only
recordset accepted by the RCU is the recordset with the sequence number
required to continue settling pending recordsets.
• Offloading Timer = maximum time between TCzA recordset transfers.
– Inflow control by MCU: If the MCU is not able to send a recordset to
the RCU within the user-specified offloading timer value, the MCU
suspends all TCzA pairs and resets the SCP condition to avoid hanging up
the system.
– Inflow control by RCU: If the RCU is not able to settle a recordset
within the user-specified offloading timer value, the RCU suspends all
TCzA pairs and resets the channel-command-retry condition to avoid
hanging up the MCU.
Concurrent Copy (CC). CC operations utilize a different cache sidefile than
TCzA. Table 2-4 also shows the sidefile threshold values for CC operations and
describes the actions that occur when each threshold is reached. Storage
systems performing TCzA in combination with CC must have sufficient cache
installed to handle the increased sidefile activity. If a “sidefile puncture”
condition occurs (CC sidefiles reach 10% over threshold), the CC session
having the highest sidefile usage is terminated.
Important:
SIM reporting for sidefile conditions is disabled when mode 118 is
OFF
(Table 2-2).