HP XP P9500 Storage - Manual (Page 11)

Main and remote control units
The primary and secondary systems are often referred to as the MCU (primary system) and RCU
(secondary system). MCU is the main control unit, RCU is the remote control unit.
MCUs control the primary storage volume (P-VOL) and the following operations:
•
Host I/O operations to the P-VOL
•
Master journal operations
•
Initial copy and update copy operations between the P-VOL and secondary volume (S-VOL).
RCUs control the secondary storage volume (S-VOL) and the following operations:
•
Issue read-journal commands to the MCU.
•
Manage the copying of journal data from master to restore journal
•
Manage the copying of restore journal data to S-VOL
•
Assist in managing pair status and configuration (for example, rejects write I/Os to the S-VOLs).
Pair volumes
Original data is stored in the P-VOL and the remote copy is stored in the S-VOL. The pair can be
paired, split, resynchronized, and returned to the simplex state. When synchronized, the volumes
are paired; when split, new data sent is to the P-VOL but held from the S-VOL. When resynchronized,
changed data is copied to the S-VOL. When a disaster occurs, production operations can be
transferred to the S-VOL. When the primary site is functional again, operations and data can be
transferred and copied back to the P-VOL.
The P-VOL remains available to the host for read and write I/O operations. The secondary system
rejects write I/Os for the S-VOL, unless the write-enable option is specified for the S-VOL. Then,
write I/O is allowed to the S-VOL while the pair is split. In this instance, S-VOL and P-VOL track
maps keep track of differential data and uses it to resynchronize the pair.
See “Duplicating VOLSERs” (page 28) for further information on volume requirements and support.
NOTE: When you configure a journal pair, you have to specify the serial numbers of primary
storage systems and secondary storage systems. You have to specify the different serial numbers
of the primary storage system and the secondary storage system for the same journal pair. If you
have to specify the same serial number, contact HP Technical Support.
Journal volumes
Journal volumes are required on the primary and secondary systems.
•
Updates to the P-VOL are copied to the master journal volume in the primary system. See the
illustration in
“Journals” (page 11) .
•
Master journal data is copied to the restore journal volume on the secondary system.
•
Journal volumes can have different volume sizes and different RAID configurations.
•
Journal data is stored sequentially and separately into each journal volume in the same journal.
For information on planning journal volumes, see “Sizing journal volumes” (page 24) .
NOTE: If a path is defined from a host to a volume, you cannot register the volume as a journal
volume.
Journals
Journals help you manage data consistency between multiple P-VOLs and S-VOLs. A journal consists
of two or more data volumes and journal volumes.
Hardware and software components 11