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Chapter 8: Using the Synchronous Mirroring Premium Feature 143
Because of the critical nature of the data being stored, do not use RAID Level 0 as the
RAID level of mirror repository volumes. The required size of each volume is
128 MB, or 256 MB total for both mirror repository volumes of a dual-controller
storage array. In previous versions of the Synchronous Mirroring premium feature,
the mirror repository volumes required less disk storage space and needed to be
upgraded to use the maximum amount of mirror relationships.
Mirror Relationships
Before you create a mirror relationship, you must enable the Synchronous Mirroring
premium feature on both the primary storage array and the secondary storage array.
You must also create a secondary volume on the secondary site if one does not already
exist. The secondary volume must be a standard volume of equal or greater capacity
than the associated primary volume.
When secondary volumes are available, you can establish a mirror relationship in the
storage management software by identifying the primary volume and the storage
array that contains the secondary volume.
When you first create the mirror relationship, a full synchronization automatically
occurs, with data from the primary volume copied in its entirety to the secondary
volume.
Data Replication
The controllers manage data replication between the primary volume and the
secondary volume. This process is transparent to host machines and applications. This
section describes how data is replicated between the storage arrays that are
participating in Synchronous Mirroring. This section also describes the actions taken
by the controller owner of the primary volume if a link interruption occurs between
storage arrays.
Write Modes
When the controller owner of the primary volume receives a write request from a
host, the controller first logs information about the write to a mirror repository
volume, and then writes the data to the primary volume. The controller then initiates a
remote write operation to copy the affected data blocks to the secondary volume at the
secondary storage array.
The Synchronous Mirroring premium feature provides two write mode options that
affect when the I/O completion indication is sent back to the host: Synchronous and
Asynchronous.
Synchronous Write Mode
Synchronous write mode provides the highest level security for full data recovery
from the secondary storage array in the event of a disaster. Synchronous write mode
does reduce host I/O performance. When this write mode is selected, host write
requests are written to the primary volume and then copied to the secondary volume.
After the host write request has been written to the primary volume and the data has
been successfully copied to the secondary volume, the controller removes the log