DELL 34XX User Manual

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:

– A full synchronization at the lowest synchronization priority level takes approximately eight times 

as long as a full synchronization at the highest synchronization priority level.

– A full synchronization at the low synchronization priority level takes approximately six times as 

long as a full synchronization at the highest synchronization priority level.

– A full synchronization at the medium synchronization priority level takes approximately three-and-

a-half times as long as a full synchronization at the highest synchronization priority level.

– A full synchronization at the high synchronization priority level takes approximately twice as long 

as a full synchronization at the highest synchronization priority level.

• When the replicated virtual disk pair is in a Synchronization in Progress state, all host write data is 

copied to the remote system. Both RAID controller module I/O bandwidth and I/O latency can affect 

host write performance. Host read performance is not affected by the replication relationship.

• The time that it takes for data to be copied from the primary virtual disk to the secondary virtual disk 

might impact overall performance. This impact is primarily caused by the delay and system resource 

required for copying data to the remote replication. Some delay might also occur because of the limit 

to the number of simultaneous writes.

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