Data Replication - IBM System Storage DS3000 Programming Manual

create a secondary logical drive on the secondary site if one does not already exist. The secondary logical
drive must be a standard logical drive of equal or greater capacity than the associated primary logical
drive.
When secondary logical drives are available, you can establish a mirror relationship in the storage
management software by identifying the primary logical drive and the storage subsystem that contains
the secondary logical drive.
When you first create the mirror relationship, a full synchronization automatically occurs, with data from
the primary logical drive copied in its entirety to the secondary logical drive.

Data Replication

The controllers manage data replication between the primary logical drive and the secondary logical
drive. This process is transparent to host machines and applications. This section describes how data is
replicated between the storage subsystems that are participating in Enhanced Remote Mirroring. This
section also describes the actions taken by the controller owner of the primary logical drive if a link
interruption occurs between storage subsystems.
Write Modes
When the controller owner of the primary logical drive receives a write request from a host, the controller
first logs information about the write to a mirror repository logical drive, and then writes the data to the
primary logical drive. The controller then initiates a remote write operation to copy the affected data
blocks to the secondary logical drive at the secondary storage subsystem.
The Enhanced Remote Mirroring feature provides two write mode options that affect when the I/O
completion indication is sent back to the host: Synchronous and Asynchronous.
Metro Mirror
Metro mirror provides the highest level security for full data recovery from the secondary storage
subsystem in the event of a disaster. Metro mirror does reduce host I/O performance. When this write
mode is selected, host write requests are written to the primary logical drive and then copied to the
secondary logical drive. After the host write request has been written to the primary logical drive and the
data has been successfully copied to the secondary logical drive, the controller removes the log record on
the mirror repository logical drive. The controller then sends an I/O completion indication back to the
host system. Metro mirror is selected as the default value and is the recommended write mode.
Global Copy
Global copy offers faster host I/O performance but does not guarantee that a copy operation has
successfully completed before processing the next write request. When you use Global copy, host write
requests are written to the primary logical drive. The controller then sends an "I/O complete" indication
back to the host system, without acknowledging that the data has been successfully copied to the
secondary (remote) storage subsystem.
When using Global copy, write requests are not guaranteed to be completed in the same order on the
secondary logical drive as they are on the primary logical drive. If the order of write requests is not
retained, data on the secondary logical drive might become inconsistent with the data on the primary
logical drive. This event could jeopardize any attempt to recover data if a disaster occurs on the primary
storage subsystem.
8-3
Chapter 8. About Enhanced Remote Mirroring premium feature