Dell PowerVault MD3420 - Manual (Page 93)

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.
Remote Replication Group
After your Remote Replication premium feature is activated on both local and remote storage arrays, you
must create Remote Replication groups on the local storage array.
A replication group contains at least one replicated virtual disk pair, one on the local storage and one on
the remote storage array, that share data synchronization settings. Multiple replicated pairs can reside in a
replication group, but each pair can only be a member of one Remote Replication group.
The following attributes also apply to a Remote Replication group:
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