Dell PowerVault MD3260i - Manual (Page 86)

completes, the system uses the point-in-time images on the secondary virtual disk to ensure that the data is maintained
in a consistent state during subsequent synchronization operations to the secondary virtual disk.
Replication Pairs And Replication Repositories
Replicated pairs, comprising of a primary and secondary virtual disk, contain identical data copies as a result of data
synchronization. Replication repository virtual disks are used to manage replication data synchronization and are
required for both the primary virtual disk and secondary virtual disk in a replicated pair.
A replication repository consists of the following data:
• Resynchronization and recovery point images for primary and secondary virtual disks.
• Tracking data from the primary virtual disk that occurs in between synchronization intervals. This data is also
written to the secondary virtual disk in case a reversal of the primary/secondary virtual disk role should occur.
Differences Between Remote Replication And Remote Replication
(Legacy) Features
The standard Remote Replication premium feature uses a point-in-time snapshot image to capture the state of the
source virtual disk and only writes data that has changed since the last point-in-time image. The Remote Replication
(Legacy) premium feature reproduces every data write to the local (primary) virtual disk on the remote (secondary)
virtual disk through a Fibre Channel-only connected configuration. While not producing a fully synchronized data view
between both storage arrays, standard Remote Replication offers a much faster replication solution that can run on both
iSCSI and Fibre Channel configurations.
NOTE: Both remote and local storage arrays must of the same data protocol. Replication between Fibre Channel
and iSCSI storage arrays is not supported.
Standard Remote Replication creates a repository virtual disk for each replicated pair, increasing throughput between
local and remote arrays. Remote Replication (Legacy) uses a single repository virtual disk for all replicated pairs.
Physical distance between local and remote storage arrays is unlimited using the Standard Remote Replication premium
feature. Remote Replication (legacy) is limited to approximately 10 Km (6.2 miles) between local and remote storage
arrays.
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 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
86