Dell 38XX, 34XX, 32XX, 36XX manual Resynchronization

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.

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