Dell PowerVault MD3260i - Manual (Page 98)

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.
Creating A Remote Replicated Pair
Before you create any replication relationships, virtual disks must exist at both the primary site and the secondary site.
The virtual disk that resides on the local storage array is the primary virtual disk. Similarly, the virtual disk that resides on
the remote storage array is the secondary virtual disk. If neither the primary virtual disk nor the secondary virtual disk
exist, you must create these virtual disks. Keep these guidelines in mind when you create the secondary virtual disk:
• The secondary virtual disk must be of equal or greater size than the primary virtual disk.
• The RAID level of the secondary virtual disk does not have to be the same as the primary virtual disk.
Use these steps to create the virtual disk.
1. Enable the Remote Replication (legacy) premium feature.
2. Activate the Remote Replication (legacy) premium feature.
3. Determine candidates for a remote replicated pair.
4. Create the remote relationship.
Performance Considerations
Keep these performance considerations in mind when you create replication relationships:
• The RAID controller module owner of a primary virtual disk performs a full synchronization in the background
while processing local I/O writes to the primary virtual disk and associated remote writes to the secondary
virtual disk. Because the full synchronization diverts RAID controller module processing resources from I/O
writes, full synchronization can have a performance impact to the host application.
• To reduce the performance impact, you can set the synchronization priority level to determine how the RAID
controller module owner will prioritize the full synchronization relative to other I/O activity. To set the
synchronization priority level, consider these guidelines:
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