HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)
Consistency G roups
The Continuous Access Asynchronous consistency groups have the following requirements:
• Each Continuous Access Asynchronous pair must be assigned to one and only one consistency
group.
• The maximum number of pairs in one consistency group is 4,096.
• The maximum number of consistency g roups established for one MCU-RCU pair is 128 (00-7F) .
The RCU supports a maximum of 128 groups. This limit includes both Continuous Access
and
TrueCopy for Mainframe groups (for example, 64 Continuous Access + 64 TrueCopy for
Mainframe).
• A Continuous
Access c onsistency group must consist only of Continuous Acc ess Asynchronous
pairs (no Continuous Access Synchronous pairs).
• Only Continuous Access Asynchronous pairs that belong to the same CLPR can be allocated
to one Continuous Access c onsistency group. For further information on CLPRs, see the HP
StorageWorks XP24000 Disk/Cache Partition User’s Guide.
Accessing Continuous Access P-VOLs and S-VOLs
To ensure maximum data integrity during no rmal Continuous Access operations, the RCU rejects all write
operations issued by a host to a Continuous Access S-VOL. If you need write access to a Continuous Access
S-VOL, you must split the pair using the S-VOL write option (see “S-VOL Write Option” on page 20).
When you resynchronize (resync) the split pair, the RCU will send the S-VOL differential data to the MCU
to ensure proper resynchronization of the pair.
Cacheand NonvolatileStorage (NVS)
Cacheand nonvolatilestorage (NVS)must beoperablefor both theMCU andRCU of aContinuous
Access pair. If not, the Continuous Access paircreate operation will fail. The remote storage system cache
should be
configured to adequately support the Continuous Access remote copy workloads, a s well as
any loc
al
workload activity.
Host Failover Software
The storage system program products do not provide any host failover functions for disaster recovery.
Host failover sof tware is a critical component of any disaster recovery effort. When an MCU fails to
maintain synchronization of a Continuous Access pair, the MCU generates sense information. It is
very importantthat thisinformation be transferredtothe remotesitevia thehostfailoversoftwarefor
effective disaster recovery.
Dupl
icate Volumes
Since the contents of the P-VOL and S-VOL of a Continuous Access pair are identical, the S-VOL can
be
co
nsidered a duplicate of the P -VOL. Since the host op erating system (OS) does not allow duplicate
volumes, the host system administrator must take precautions to prevent system problems related to
duplicate volumes. You must define the Continuous Access S-VOLs so they do not auto-mount or come
onl
inetothe same host at thesame timeasthe P-VOLs (see WARNING below).
Continuous Access does not a llow the S-VOL to be online (except when the pair is split). If the S-VOL is
online, the Continuous Access paircreate operation will fail.
WAR
NING:
If the Continuous Access S-VOLs are physically attached to the same host server(s) as the
Continuous Access P-VOLs, the following problem can occur:
• When a Continuous Access pair is released, the old S -VOL is usually offline. If the host is then
res
tarted, the system administrator may be offered both volumes and asked which volume should
be
left offline. This can be confusing and is prone to error.
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