Dell PowerVault MD3420 - Manual (Page 102)

RAID controller module owner of the primary virtual disk but will not accept host write requests. Hosts
are able to read from the secondary virtual disk, which appears as read-only.
In the event of a disaster or a catastrophic failure at the primary site, you can perform a role reversal to
promote the secondary virtual disk to a primary role. Hosts then are able to read from and write to the
newly promoted virtual disk, and business operations can continue.
Replication Repository Virtual Disks
A replication repository virtual disk is a special virtual disk in the storage array that is created as a resource
for the RAID controller module owner of the primary virtual disk in a remote replicated pair. The RAID
controller module stores replication information on this virtual disk, including information about remote
writes that are not yet complete. The RAID controller module can use this information to recover from
RAID controller module resets and the accidental powering down of the storage arrays.
When you activate the Remote Replication (legacy) premium feature on the storage array, you create two
replication repository virtual disks, one for each RAID controller module in the storage array. An individual
replication repository virtual disk is not needed for each remote replication.
When you create the replication repository virtual disks, you specify the location of the virtual disks. You
can either use existing free capacity, or you can create a disk group for the virtual disks from
unconfigured capacity and then specify the RAID level.
Because of the critical nature of the data being stored, do not use RAID Level 0 as the RAID level of
replication repository virtual disks. The required size of each virtual disk is 128 MB, or 256 MB total for
both replication repository virtual disks of a dual-RAID controller module storage array. In previous
versions of the Remote Replication (legacy) premium feature, the replication repository virtual disks
required less disk storage space and needed to be upgraded to use the maximum amount of replication
relationships.
Replication Relationships
Before you create a replication relationship, you must enable the Remote Replication (legacy) premium
feature on both the primary storage array and the secondary storage array. You must also create a
secondary virtual disk on the secondary site if one does not already exist. The secondary virtual disk must
be a standard virtual disk of equal or greater capacity than the associated primary virtual disk.
When secondary virtual disks are available, you can establish a replication relationship in the MD storage
management software by identifying the primary virtual disk and the storage array that contains the
secondary virtual disk.
When you first create the replication relationship, a full synchronization automatically occurs, with data
from the primary virtual disk copied in its entirety to the secondary virtual disk.
Data Replication
The RAID controller modules manage data replication between the primary virtual disk and the secondary
virtual disk. This process is transparent to host machines and applications. This section describes how
data is replicated between the storage arrays that are participating in Remote Replication (legacy). This
section also describes the actions taken by the RAID controller module owner of the primary virtual disk if
a link interruption occurs between storage arrays.
Write Modes
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,
and then writes the data to the primary virtual disk. The RAID controller module then initiates a remote
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