HP XP Continuous Access Software - Manual (Page 32)

32 Continuous Access XP for the XP1024/XP128
Continuous Access XP, a LUSE P-VOL must be paired with an S-VOL of the same size and structure. For
example, if a LUSE P-VOL is connected with volumes of 1GB, 2GB, and 3GB in this order, you must
specify the LUSE volume that has exactly the same size and connection order as the S-VOL. When LUSE
volumes are assigned to a Continuous Access XP pair, the P-VOL and S-VOL must contain the same type
and number of LDEVs (for example, OPEN-3*6 to OPEN-3*6). The number of LDEV pairs in a Continuous
Access XP LUSE pair equals the number of LDEVs in each LUSE volume. When the status of all LDEV pairs
within one Continuous Access XP LUSE pair is not the same (for example, one LDEV pair is suspended),
the pair status of the Continuous Access XP LUSE pair changes to PDUB.
Continuous Access XP supports the CVS feature of the disk array, which configures custom-size LUs that are
smaller than standard-size LUs. When custom-size LUs are assigned to a Continuous Access XP pair, the
S-VOL must have the same capacity as the P-VOL.
Consistency groups
Continuous Access XP Asynchronous consistency groups have the following requirements:
• Each Continuous Access XP Async pair must be assigned to one and only one consistency group.
• Maximum number of volume pairs in one consistency group is 4,096 (entire RCU) (4,095 when the
RAID Manager command device is defined).
• Maximum number of consistency groups established for one MCU-RCU pair is 128 (00-7F). The RCU
supports a maximum of 128 groups, which includes both Continuous Access XP Async groups and
Mainframe mirroring Asynchronous groups (for example, 64 Continuous Access XP Async + 64
mainframe).
• Consists of only Continuous Access XP Async pairs (no mainframe pairs).
• Only Continuous Access XP Asynchronous pairs belonging to the same CLPR can be allocated to one
Continuous Access XP consistency group. For more information about CLPRs, see the
HP StorageWorks XP Disk/Cache Partition user guide.
Accessing Continuous Access XP P-VOLs and S-VOLs
To ensure maximum data integrity during normal Continuous Access XP operations, the RCU rejects all
write operations issued by a host to a Continuous Access XP S-VOL. If you need write access to a
Continuous Access XP S-VOL, split the pair using the S-VOL write option (see ” S-VOL write option ” on
page 20). When you resume (resync) the split pair, the RCU sends the S-VOL cylinder bitmap to the MCU
to ensure proper resynchronization of the pair.
Cache and nonvolatile storage (NVS)
Cache and nonvolatile storage (NVS) must be operable for both the MCU and RCU of a Continuous
Access XP volume pair. If not, the Continuous Access XP paircreate operation fails. Configure the remote
disk array cache to adequately support Continuous Access XP remote copy workloads and local workload
activity.
Host failover software
Host failover software is a critical component of any disaster recovery effort. When an MCU fails to
maintain synchronization of a Continuous Access XP pair, the MCU generates sense information. This
information must be transferred to the remote site via the host failover software for effective disaster
recovery.
Duplicate volumes
Because the contents of the P-VOL and S-VOL of a Continuous Access XP pair are identical, the S-VOL is
considered a duplicate of the P-VOL. Because the host operating system 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 XP S-VOLs so they do not auto-mount or come online to
the same host at the same time as the P-VOLs (see the following CAUTION).
Continuous Access XP does not allow the S-VOL to be online (except when the pair is split). If the S-VOL is
online, the Continuous Access XP paircreate operation fails.