Dell PowerVault MD 32XX/36XX Series Storage Arrays CLI Guide

When processing write requests, the primary RAID controller module might be able to write to the primary virtual disk, but a link interruption might prevent communication with the remote (secondary) RAID controller module.

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.