The lowdown on replication appliances


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Implementation options
There are five general configuration options for implementing a replication appliance:

  • Pass-through. The replication appliance sits between the server and storage, and presents the storage array LUNs to the host. I/O passes through the appliance with only write I/Os copied by the appliance. This approach doesn't require host-based agents, but it does requires more time to set up and configure; it may also present problems for path failover software (see "

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  • Pass-through" next page).
  • Data Mover. The appliance replicates data from one storage array to another in the background with minimal host or application disruption. This implementation is ideal for data migrations from one storage array to another without the use of host-based agents (see "Data mover" next page).
  • Split Write over TCP/IP. An agent runs on the host and sends copies of the write I/Os to the replication appliance over the host's TCP/IP interface. This is a cost-effective approach that works well for low-end or lightly used servers that aren't connected to a FC SAN, but need the same replication benefits offered to FC- attached servers (see "Split Write over TCP/IP" ).
  • Split Write to Fibre Channel Target. An agent runs on the host and copies write I/Os to the FC targets presented by the replication appliance to the host's FC interface. This technique offloads the I/O processing from the server to the FC host bus adapter (HBA), and sends data to the LUNs presented by the replication appliance without directly introducing the appliance in the data path between the server and the storage (see "Split Write to Fibre Channel target").
  • Split Write at Fibre Channel Switch. The replication appliance sends commands to a service running on the FC director to copy and send write I/Os to the replication appliance. Unlike other Split Write methods, this approach doesn't require a host-based agent as commands are sent from the replication appliance to the FC director over FC (see "Split Write at Fibre Channel switch", next page).
FalconStor's IPStor, McData's UltraNet Replication Appliance and Sun's MirrorStore each offer pass-through as one of the ways users may implement their replication appliance for continuous replication. This option may be selected when one or more of the following circumstances exist:
  • Host-based replication products don't support the operating system or the version of the operating system on the host.
  • Placing host-based replication software on the hosts is problematic in the environment.
  • The replication requires an approach that's minimally disruptive to the host.
  • Users are moving from one vendor's storage array to another vendor's storage array.
  • Users want to place data on a larger sized volume on a new array than on the existing volume on the current array.

This was first published in November 2005

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