HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

When the S-VOL write enable pairsplit option is used, the RCU keeps track of S-VOL updates and sends
the
S-VOL differential data to the MCU when the split pair is resynchronized. In this case, the MCU
merges the P-VOL and S-VOL differential data to identify the out-of-sync tracks.
For additional flexibility, Continuous Access p rovides the following options for initial copy:
• The number of tracks option allows you to specify how many tracks are copied simultaneously by
the
Continuous Access initial copy operation when creating/resynchroniz ing a Continuous Access
pair. This option can only be specified using the remote console software (not RAID Mana ger).
• The initial copy priority option allows you to specify the order in which the initial copy operations
are performed when creating/resynchronizing multiple Continuous Access pairs. This option can
only be specified using the Continuous Access software (not RAID Manager).
• The Maximum initial copy activity option allows you to specify the maximum number of concurrent
initial copy operations that each MCU can perform (not pair-specific) . This option can only be
specified using the Continuous Access software (not RAID Manager).
Update Copy Operation
An update copy operation occurs when the host issues a write I/O operation to the P-VOL of a
Continuous Access pair. The update copy operation duplicates the P-VOL write I/O at the S-VOL to keep
the
pair sy
nchronized. Continuous Access provides two modes for update copy operations: synchronous
and
asynchronous. The update copy mode is specified when you add a Continuous Access pair and
cannot
be changed.
For the syn
chronous update copy mode, the MCU ensures that the P-VOL and S-VOL are synchronized at
all
times. The MCU does not return a final ending status for the P-VOL write I/O until both the P-VOL write
and
its associated update copy operation at the RCU are complete. For the synchronous m o d e, the MCU
starts th
e
update copy operation when it receives:
• a write command whose data leng th 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 dat
a
to the last track for the command.
Note: 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
indepen
dent
of the host I/O processes. These updates along with their associated control information
are called Continuous Access Asynchronous recordsets. The RCU stores the 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 Continuous Access update sequence
inform
ation. For further information on Continuous Access Asynchronous recordset operations, see
“Continuous Access Asynchronous Recordset Operations” on page 22.
Priority of initial and update copy: In both Continuous Access Synchronous and Continuous Access
Asynchronous, update copy has a higher priority than initial copy. However, initial copy is executed
based on the copy pace (3 or 1 5 tracks); therefore, update copy must wait for this interval if initial copy
is
bei
ng
executed. For example, if the copy pace is 15 tracks, the update copy may wait up to 15 tracks
(1
cy
linder). In the case of 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 Continuous Access Volumes
When an MCU receives a read command for a Continuous Access P-VOL, the MCU comp letes 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 Continuous Access S-VOL for recovery.
When an MCU receives a write command for a 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 a Continuous
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