Update Copy Operations; Read And Write I/O Operations For Continuous Access Xp Volumes - HP StorageWorks XP12000 User Manual

Update copy operations

Update copy operations occur when the host issues a write I/O operation to the P-VOL of an established
Continuous Access XP pair. Update copy operations duplicate P-VOL write I/Os at the S-VOL to keep
the volume pair synchronized. Continuous Access XP provides two modes for update copy operations:
synchronous and asynchronous. Update copy mode is specified when you add a Continuous Access XP
pair and is unchangeable for the life of the pair.
For synchronous update copy mode, the MCU ensures the P-VOL and S-VOL are synchronized at all times.
When using fence level "data," the MCU does not return final ending status for the P-VOL 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
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.
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Continuous Access XP for the Array
Continuous Access XP Asynchronous recordset operations.