HP StorageWorks XP Remote Web Console Software - Manual (Page 14)

write I/O until the outcome of the P-VOL write and its associated update copy operation at the
RCU are known. For synchronous mode, the MCU starts the update copy operation when it receives:
•
A write command whose data length is short enough not to pass the track-end
or
•
Write data to the track-end, whose total length is long enough to pass the track-end
or
•
Write data to the last track for the command
NOTE: No matter how long the write data is, update copy is executed for each track.
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
“XP Continuous Access Asynchronous recordset operations”
(page 17) .
Priority of initial and update copy: In both XP Continuous Access Synchronous and XP Continuous
Access 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 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 the P-VOL. The MCU will not read the XP Continuous Access S-VOL over the link if
a failure renders the P-VOL inaccessible.
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 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 XP Continuous Access 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
“XP
14 Overview of XP Continuous Access operations