The lowdown on replication appliances


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Split Writes
To replicate data on a continuous basis and alleviate user concerns about in-band implementations, vendors offer a host agent that performs a copy of the write I/O--a Split Write--that gets sent to their replication appliance. This Split Write serves two purposes. First, it eliminates the need to put the appliance directly in the data path between the server and the storage array. Second, it takes advantage of the replication appliance's connection to the FC SAN and its ability to present LUNs to the host's FC interface.

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There are two ways to implement the Split-Write feature: at the host or at the fabric layer. Loading the agent on the host may impact the host in a couple of ways. From an implementation perspective, it may require a reboot of the server to load the driver that copies the write I/Os or to discover the LUNs presented by the replication appliance. And copying the write I/Os may impact application performance, although vendors universally claim these agents only impact applications that have high write I/O transaction rates.

For servers without FC connections, Topio's TDPS offers configurations that support sending the Split Write over a TCP/IP connection. TDPS copies the write and either stores the write to cache or disk before transmitting the write I/Os over IP. In the event of a loss of IP connectivity, the writes are stored until the link is restored. This option gives servers that have either internal disk or SCSI-attached external disk replication capabilities similar to those FC disk offers.

Ease of implementation and performance impact will be the two key factors when choosing between a TCP/IP or FC connection to send the Split Writes. Using a TCP/IP connection is easier to implement, but it generates additional CPU and memory overhead on the host and more traffic on the network, so it should be used only for lower performance applications. Conversely, sending the Split Writes through the server's FC HBA can take more time to set up, but it offloads the I/O processing to the HBA, minimizing application impact. But this method of re-introducing a host-based agent results in the same issues as host-based replication methods from NSI Software Inc., Softek Storage Solutions Corp. and Symantec Corp.--some host operating systems agents aren't available and, in other cases, implementation may be disruptive to hosts.

To address these shortcomings, replication appliances now interact with services that run at the fabric level on FC director blades. Both Kashya's KBX5000 and Xiotech's TimeScale replication appliances work with Cisco Systems Inc.'s SANTap Service that runs as a feature on Cisco's Storage Services Module (SSM) line cards. By sending SCSI-FCP commands over the FC interface from the replication appliance to the SANTap Service on Cisco's SSM, these implementations can capture and copy all reliable write I/Os to the replication appliance. Cisco is currently the only vendor to support this type of feature. But replication appliance vendors say they're working with Brocade Communications Systems Inc. and McData, who plan to offer similar functionality on their FC directors.

The main advantage of this approach is that write I/Os may be captured without virtualizing a portion of the storage environment or deploying host agents. But it also has its downsides. Configuring specific point-in-time recoverable images for applications is more difficult because without an agent on the host there's no certain way to stop and start I/Os to ensure the creation of a recoverable image. And some replication appliance vendors are still not entirely clear on how to isolate write I/Os from specific servers or applications, so the practical benefits of this feature may be minimal in the near term. Still, this approach seems to have the inside track to become the preferred method to implement data replication on a wider scale in large, heterogeneous networked storage environments.

This was first published in November 2005

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