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

the synchronous mode, the MCU starts the update copy operation when it receives any of the
following:
•
A write command whose data length is short enough not to pass the track-end
•
Write data to the track-end whose total length is long enough to pass the track-end
•
Write data to the last track for the command
No matter how long the write data is, update copy is executed for each track.
For the asynchronous update copy mode, the MCU stores the P-VOL updates along with additional
control information in cache, and sends the updates and control information to the RCU completely
independent of the host I/O processes. These updates along with their associated control information
are called XP Continuous Access Asynchronous recordsets. The RCU stores the XP Continuous
Access Asynchronous recordsets in cache and performs the updates to the S-VOLs in the same
order as they were performed at the MCU(s) according to the XP Continuous Access update
sequence information.
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
18 About XP Continuous Access Operations