HP StorageWorks XP Remote Web Console Software - Manual (Page 18)

Priority of initial and update copy: In both XP Continuous Access Synchronous and XP Continuous
Access Asynchronous, update copy has a higher priority than initial copy. However, initial copy
is executed based on the copy pace (3 or 15 tracks); therefore, update copy must wait for 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). In the case of XP Continuous Access 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 XP Continuous Access Volumes
When an MCU receives a read command for an XP Continuous Access P-VOL, the MCU completes
the read from either cache or the P-VOL. If the read fails, the redundancy provided by RAID
technology recovers the failure. The MCU does not read the XP Continuous Access S-VOL for
recovery.
When an MCU receives a write command for an XP Continuous Access 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 an XP Continuous Access 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 an XP Continuous Access P-VOL with pair status,
the user-selected update copy mode (synchronous or asynchronous) determines the following
sequence of events:
•
Synchronous Mode: The MCU performs the write operation on the P-VOL, starts the update
copy operation for the S-VOL, and then reports the final ending status to the host only after
the update copy operation is complete. 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 will regard
that write operation to the P-VOL as failed. If a failure occurs at the P-VOL or the S-VOL, the
corresponding volume of the XP Continuous Access pair will not commit to the update to
maintain exact synchronization of the volumes.
•
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 the XP Continuous Access Asynchronous recordset information and maintains
sequence-based data consistency for the S-VOLs. If the P-VOL write operation fails, the MCU
reports a unit check and does not create the XP Continuous Access Async recordset for this
operation. If the update copy operation fails, the RCU suspends either the affected pair or all
XP Continuous Access Asynchronous pairs in the consistency group, depending on the type
of failure. When the suspended XP Continuous Access pair or group is resynchronized
(pairresync), the MCU and RCU negotiate the resynchronization of the pair(s). See
“XP
Continuous Access Suspend Types” (page 34) for further information on suspended XP
Continuous Access Asynchronous pairs.
The RCU does not allow an XP Continuous Access S-VOL to be online (mounted) during normal XP
Continuous Access operations and rejects all host-requested write I/O operations for an S-VOL.
The special XP Continuous Access S-VOL write enable option enables write access to an S-VOL
while the pair is split (see
“S-VOL Write Option” (page 18) ). The S-VOL write option can only be
enabled when you split the pair from the MCU.
S-VOL Write Option
For additional flexibility, XP Continuous Access provides an S-VOL write option that enables write
access to the S-VOL of a split XP Continuous Access pair. The S-VOL write option can be selected
by the user during the pairsplit-r operation and applies only to the selected pair(s). The S-VOL write
option can be accessed only when you are connected to the MCU. When you resync a split XP
Continuous Access pair which has the S-VOL write option enabled, the RCU sends the S-VOL
18 About XP Continuous Access Operations