CLI Guide

In this case, the remote write operation cannot be completed to the secondary virtual disk, and the

primary virtual disk and the secondary virtual disk are no longer correctly replicated. The primary RAID

controller module transitions the replicated pair into an Unsynchronized state and sends an I/O

completion to the primary host. The primary host can continue to write to the primary virtual disk, but

remote writes do not take place.

When communication is restored between the RAID controller module owner of the primary virtual disk

and the RAID controller module owner of the secondary virtual disk, a resynchronization takes place. This

resynchronization happens automatically, or it must be started manually, depending on which write

mode you chose when setting up the replication relationship. During the resynchronization, only the

blocks of data that have changed on the primary virtual disk during the link interruption are copied to the

secondary virtual disk. After the resynchronization starts, the replicated pair transitions from an

Unsynchronized status to a Synchronization in Progress status.

The primary RAID controller module also marks the replicated pair as unsynchronized when a virtual disk

error on the secondary side prevents the remote write from completing. For example, an offline

secondary virtual disk or a failed secondary virtual disk can cause the remote replication to become

unsynchronized. When the virtual disk error is corrected (the secondary virtual disk is placed online or

recovered to an Optimal status), then synchronization is required. The replicated pair then transitions to a

Synchronization in Progress status.

Resynchronization

Data replication between the primary virtual disk and the secondary virtual disk in a replication

relationship is managed by the RAID controller modules and is transparent to host machines and

applications. When the RAID controller module owner of the primary virtual disk receives a write request

from a host, the RAID controller module first logs information about the write to a replication repository

virtual disk. The RAID controller module then writes the data to the primary virtual disk. The RAID

controller module then initiates a write operation to copy the affected data to the secondary virtual disk

on the remote storage array.

If a link interruption or a virtual disk error prevents communication with the secondary storage array, the

RAID controller module owner of the primary virtual disk transitions the replicated pair into an

Unsynchronized status. The RAID controller module owner then sends an I/O completion to the host

sending the write request. The host can continue to issue write requests to the primary virtual disk, but

remote writes to the secondary virtual disk do not take place.

When connectivity is restored between the RAID controller module owner of the primary virtual disk and

the RAID controller module owner of the secondary virtual disk, the virtual disks must be resynchronized

by copying the blocks of data that changed during the interruption to the secondary virtual disk. Only the

blocks of data that have changed on the primary virtual disk during the link interruption are copied to the

secondary virtual disk.

CAUTION: Possible loss of data access – Any communication disruptions between the primary

storage array and the secondary storage array while resynchronization is underway could result

in a mix of new data and old data on the secondary virtual disk. This condition would render the

data unusable in a disaster recovery situation.