HP XP Racks - Manual (Page 9)

Main and remote control units
Like Continuous Access Synchronous, ESAM replication relationships exist at the Logical Control
Unit (LCU) level within the storage systems.
•
Primary storage system LCUs containing the production volumes to be replicated are called
MCUs (main control units).
•
Secondary storage system LCUs containing the copy volumes are called remote control units
(RCUs).
Normally the MCU contains the P-VOLs and the RCU contains the S-VOLs.
The MCU communicates with the RCU via the data path. You can simultaneously set P-VOL and
S-VOL in the same storage system if the volumes are used by different pairs. In this case, the CU
can function simultaneously as an MCU for the P-VOL and as an RCU for the S-VOL.
The MCU is often referred to as the primary storage system in this document; the RCU is often
referred to as the secondary storage system.
Pair volumes
Original data from the host is stored in the P-VOL; the remote copy is stored in the S-VOL. Data is
copied as it is written to the P-VOL; new updates are copied only when the previous updates are
acknowledged in both primary and secondary volumes.
Once a pair is created, you can do the following:
•
Split the pair, which suspends copy activity.
•
Resynchronize the pair, which restores and maintains synchronization.
•
Delete the pair, which removes the pair relationship, though not the data.
Data paths
The physical links between the primary and secondary storage systems are referred to as the "data
path." These links include the Fibre Channel interface cables and switches. ESAM commands and
data are transmitted through the data path. The data path links the primary and secondary storage
systems through two types of Fibre Channel ports, Initiator and RCU Target ports.
Because paths are one-directional, and ESAM communicates in both directions, a minimum of two
data paths are needed; however, HP requires a minimum of two in each direction for greater
support and security of the data. A maximum of eight data paths in each direction are supported.
Therefore, the maximum number of logical paths between any two storage systems is sixteen (eight
forward and eight reverse).
Quorum disk
The quorum disk is a continuously updated volume that contains information about the state of data
consistency between the P-VOL and S-VOL. The information is used by ESAM in the event of failure
to direct host operations to the secondary volume. The quorum disk is located in an externally
attached storage system.
Multipath software
Multipath software distributes the loads among the paths to the current production volume. For
ESAM, the multipath software duplicates the paths between the host and P-VOL, so that the paths
are in place between the host and the S-VOL also.
If a failure occurs in the data path to the primary storage system, or with the primary storage
system, the multipath software transfers host operations to the S-VOL in the secondary storage
system.
ESAM components 9