Dell PowerVault MD3260i - Manual (Page 96)

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 write operation to copy the affected data
blocks to the secondary virtual disk at the secondary storage array.
The Remote Replication (legacy) premium feature provides two write mode options that affect when the I/O completion
indication is sent back to the host: Synchronous and Asynchronous.
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