Dell PowerVault MD3600f Series, Command Line Interface Manual

Page: 97 / 328

Using the Remote Replication Premium Feature
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When using Asynchronous write mode, write requests are not guaranteed to
be completed in the same order on the secondary virtual disk as they are on
the primary virtual disk. If the order of write requests is not retained, data on
the secondary virtual disk might become inconsistent with the data on the
primary virtual disk. This event could jeopardize any attempt to recover data
if a disaster occurs on the primary storage array.
Write Consistency Mode
When multiple replication relationships exist on a single storage array and
have been configured to use Asynchronous write mode and to preserve
consistent write order, they are considered to be an interdependent group that
is in the Write consistency mode. The data on the secondary, remote storage
array cannot be considered fully synchronized until all of the remote
replications that are in the Write consistency mode are synchronized.
If one replication relationship in the group becomes unsynchronized, all of
the replication relationships in the group become unsynchronized. Any write
activity to the remote, secondary storage arrays is prevented to protect the
consistency of the remote data set.
Link Interruptions or Secondary Virtual Disk Errors
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
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