HP StorageWorks XP Remote Web Console Software - Manual (Page 30)

30 Hitachi TrueCopy z/OS for HP XP12000/XP10000 and SVS 200 storage systems
NOTE: The MCU does not remove the sidefile entry for a recordset from its cache until it receives an I/O
completion signal (device end) from the RCU. This is true even if the MCU and RCU are connected through
a channel extender product. If a recordset is lost in transmission from the MCU to the RCU, the MCU’s
cylinder bitmap ensures that the missing recordset is identified and resent to the RCU.
Selecting and settling recordsets at the RCU
The RCU selects the recordset to be promoted to formal data (or “settled”) as follows:
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 (for example, 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 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