HP XP Continuous Access Software - Manual (Page 20)

20 Continuous Access XP for the XP1024/XP128
For asynchronous update copy mode, the MCU stores P-VOL updates with additional control information
in cache, and sends updates and control information to the RCU completely independent of host I/O
processes. These updates and their associated control information are called recordsets. The RCU stores
recordsets in cache and performs updates to S-VOLs in the same order as they were performed at MCUs
according to the sequence information stored in the recordset. For more information on recordset
operations, see ” Continuous Access XP Asynchronous recordset operations ” on page 21.
Priority of initial and update copy: In both Continuous Access XP Synchronous and Continuous Access XP
Asynchronous, update copy has higher priority than initial copy. However, initial copy is executed based
on copy pace (3 or 15 tracks). Therefore, update copy must wait this interval if initial copy is being
executed. For example, if the copy pace is 15 tracks, the update copy may wait up to 15 tracks (1
cylinder). For Continuous Access XP Asynchronous, update copy is executed asynchronously, but the
same scheduling conflict can occur between the asynchronous update copy (write recordset) and initial
copy.
Read and write I/O operations for Continuous Access XP volumes
When an MCU receives a read command for a Continuous Access XP P-VOL, the MCU completes the
read from the P-VOL. The MCU will not read the Continuous Access XP S-VOL over the link if a failure
rendered the P-VOL inaccessible.
When an MCU receives a write command for a Continuous Access XP Synchronous P-VOL with COPY
status and the track has already been copied to the S-VOL, the MCU performs a synchronous update copy
operation to complete the write at the S-VOL. When an MCU receives a write command for a Continuous
Access XP Asynchronous P-VOL with COPY status and the track has already been copied to the S-VOL, the
MCU performs an asynchronous update copy operation.
When an MCU receives a write command for a Continuous Access XP P-VOL with PAIR status, the
user-selected update copy mode (synchronous or asynchronous) determines the sequence of events as
follows:
• Synchronous Mode: The MCU performs the write operation on the P-VOL, starts the update copy
operation for the S-VOL, and reports final ending status to the host only after results of the update copy
operation are known. If the P-VOL write or S-VOL update copy operation fails, the MCU reports a unit
check, and the host system and application program regard that write operation to the P-VOL as
failed.
• Asynchronous Mode: The MCU completes P-VOL write operations independently of the associated
update copy operations at the S-VOL. The RCU manages the S-VOL updates according to recordset
information and maintains sequence ordered data consistency for the S-VOLs. If the P-VOL write
operation fails, the MCU reports a unit check and does not create the Continuous Access XP
asynchronous recordset for this operation. If the update copy operation fails, the RCU optionally
suspends the affected pair or all pairs in the consistency group, depending on the type of failure.
When the suspended pair or group is resumed (pairresync), the MCU and RCU negotiate the
resynchronization of the pairs. See ” Continuous Access XP suspend types ” on page 28 for further
information about suspended Continuous Access XP Async pairs.
The RCU does not allow a Continuous Access XP S-VOL to be online (mounted) during normal Continuous
Access XP operations and rejects all host-requested write I/O operations for an S-VOL. The special
Continuous Access XP S-VOL write enable option enables write access to an S-VOL while the pair is split
(see ” S-VOL write option ” on page 20). The S-VOL write option can be enabled only when you split the
pair from the MCU.
To reduce overhead associated with Continuous Access XP remote copy operations, the XP1024/XP128
uses a special write command that is allowed only for Continuous Access XP initial and update copy
operations. This Continuous Access XP command transfers control parameters and FBA-format data,
which includes consecutive updated records in each track, using a single write operation. This capability
eliminates overhead required for performing FBA-to-CKD and CKD-to-FBA conversions.
S-VOL write option
Continuous Access XP provides an S-VOL write option that enables write access to the S-VOL of a split
Continuous Access XP pair. The S-VOL write option can be selected during the pairsplit-r operation and
applies only to the selected pairs. The S-VOL write option can be accessed only when you are connected