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

Access Asynchronous P-VOL with COPY status and the track has already been c opied to the S-VOL, the
MCU
performs an asynchronous update copy operation.
When an MCU receives a write command for a Continuous Access P-VOL with PAIR status, the
user-selected upda te c opy mode (synchronous or asynchronous) determines the sequ ence of events:
• Synchronous M ode: The MCU performs the write operation on the P-VOL, starts the update
copy operation for the S-VOL, a nd then reports the final ending status to the host only af ter 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 Continuous Access pair will decommit the update to m aintain 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 accord ing to the
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 Continuous Access Async recordset for this operation. If the update copy
operation fails, the RCU suspends either the affected pair or all Continuous Access Asynchronous
pairs in the consistency group, depending on the type of failure. When the suspended
Continuous Access pair or group is resynchronized (pairresync), the MCU and RCU negotiate the
resynchronization of the pair(s). See “Continuous Access Suspend Types”onpage30for further
information on suspended Continuous Access Asynchronous pairs.
The RCU does not allow a Continuous Access S-VOL to be online (mounted) during normal Continuous
Access operations and rejects all host-requested write I/O operations for an S-VOL. The special
Continuous Access S-VOL write enable option enables write ac cess to an S-VOL while the pair is split
(see “S-VOL Write Option” on p age 20). The S-VOL write option can only be enabled when you split
the pair
from the MCU.
S-VOL Write Option
For add
itional flexibility, Continuous Access provides an S-VOL write option which enables write access to
the
S-VOL of a split 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 wh
en
you are connected to the MCU. When you resync a split Continuous Access pair which has
the
S-VOL write option enabled, the RCU sends the S-VOL differential data to the MCU, and the MCU
merges the P-VOL and S-VOL differential data to determine which tracks are o ut-of sync. This ensures
proper resynchronization of the pair.
Difference Managem ent
The differential data (updated by write I/Os d uring a split or suspension) between the P-VOL and S-VOL
is
stored in each track bitmap. When a split/suspended pair is resynchronized (pairresync), the MCU
merges the P-VOL and S-VOL bitmaps, and the differential data is copied to the S-VOL.
The maximum number of pairs you can create is restricted. Use the number of cylinders and bitmap areas
to
calculate the maximum number of pairs that can be created in a storage system.
• The number of cylinders:
The number of cylinders depends on the capacity of the volumes (the number of cylinders set
by VLL, if you set the VLL feature) used for the pairs. Both the MCU a nd RCU are restricted. To
calculate the number of cylinders, use the following formulas:
• For OPEN-3, OPEN-8, OPEN-9, OPEN-E, OPEN-L, OPEN-K:
(↑ ((↑ (Max. LBA ÷ 96) ↑) ÷ 15) ↑)+1
•Fo
rOPEN-V:
(↑ ((↑ (Max. LBA ÷ 512) ↑)÷15) ↑)+1
The ↑…↑ symbols around a value indicate that the value should b e rounded up to the next
integer.
20
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HP StorageWorks XP Continuous Access Software Operations