HP StorageWorks XP Remote Web Console Software - Manual (Page 31)

TrueCopy for z/OS user guide 31
2. When the top of each queue contains a valid entry (recordset), the RCU selects the entry that has the
earliest time-stamp value, and then settles this recordset.
3. The RCU repeats this process to select and settle TC390A recordsets.
The following figure illustrates recordset selection and settling at the RCU. In this example, the top of the
queue contains a valid entry: S1/T1. The RCU selects recordset S1/T1 to be settled because T1 is the
earliest time-stamp. When S1/T1 is removed from the MCU queue, recordset S2 becomes the top entry,
but it is empty. When recordset S2 arrives (and its time-stamp is later than T1 and earlier than T3), the
RCU selects S2/T2 as the next recordset to be settled. The recordset selected by the RCU is marked as
“host-dirty” and treated as formal data. The time-stamp value of that recordset is promoted to the
consistency time (C/T) of the group. The RCU settles the updated records in the recordset as follows:
• If the corresponding track is in cache (track-hit), the updated records in the recordset are copied to the
existing cached track and the 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. The data is not physically moved.
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 and when an
MCU power-off sequence is initiated, and also maintain sequence numbers in periods of low host
activities.
Figure 4 Selecting and settling TrueCopy Async recordsets at the RCU
Inflow Control of Recordsets
As described in the previous sections, both the MCU and RCU create sidefiles for storing TC390A
recordsets. Because the 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
XP128/XP1024/XP10000/XP12000 uses the following parameters (specified on the TC390 Async
Option window, see ” Asynchronous Copy Option ” on page 102) for TC390A cache inflow control:
• Sidefile Threshold = maximum cache % available for use by TC390A sidefiles. Table 7 on page 33
shows the TC390A sidefile threshold values and describes the actions that occur when each threshold
is reached.