Quadric Software

Enhanced Replication with Alike DR (pre v2.0.8)

Please Note:  This article is out of date.  The contents and instructions contained in this article apply to version 2.08 and earlier of Alike DR. 

For the most recent information, please refer to the current KB article.


Alike DR Edition includes a new feature called Enhanced Replication (ER).  Enhanced Replication is similar in concept to Standard Replication, but is very different under the hood and provides much more flexibility for deployment.  The performance gains can be very significant with an ER job, especially when replicating between two local SRs or over a lower bandwidth connection.  This article will provide an overview of Enhanced Replications features and configurations.

ER Overview
ER allows you to copy a point-in-time snapshot of a VM from one system or SR to another.  Furthermore, if you select 'overwrite target' you can run this replication on a schedule.  This keeps the target VM up-to-date with the source VM's changes on a routine basis.  This can provide a hardware and SR agnostic fault tolerance for your VMs.

For example, a pooled environment may have production VMs running out of a SR located on shared storage.  With ER, you could schedule a replication of your VMs on a nightly basis to you XenServers' local storage(s).  In the event of a SAN or SR failure, you would be able to boot the VMs off of your XenServers' local storage for a rapid service recovery.  And once the production SR is back online, ER would allow you to configure replication in reverse, sending your VMs back to production.

ER jobs provide stream compression to lower the amount of bandwidth required when replicating a VM over your network.  Additionally, for repeated/scheduled replication jobs, Alike employs a differencing engine, sending only the changed data across the network.  

Because of these capabilities, another very common usage of ER is for DR purposes.  Replicating VMs from a primary pool or location to an offsite DR pool can provide a significant amount of site level protection.  Thanks to the compression and ability to only send changed data, ER can efficiently replicate even over lower bandwidth connections.

Configuring ER
Enhanced Replication leverages QuorumSoft's ABD (Alike Backup Delegate) technology, just like Enhanced Backups.  To run an ER, an ABD  must be deployed to both the source and destination Resource Pool.  To learn more about ABDs and how to deploy them, please refer to our KB article here.  If your source and destination VMs are both in the same Resource Pool, Alike will require only one ABD.  Please see the caveats section below for more details.

Once the ABDs are deployed, configuring ER is very straightforward.  You simply create a new Replication job from Alike's Job page, and follow the wizard until Step 4.  Step 4 (shown below) allows you to select your destination SR and choose the job options.  Here you can select the "Use enhanced replication".  If there is a problem, Alike will instruct you in the text above.  If everything is ready, you will see the green text highlighted below.  You can then save the job, and run it immediately or on a schedule.



Overwrite Target Option
The overwrite target option in particular is very important for replication jobs (enhanced or standard).  It tells Alike to create or replace the target VM in the replication job.  If this option is not selected, and Alike finds the target VM during the replication, the job will fail. This option is required if your replication is scheduled to run more than once.  
Please refer to the KB article on this topic here.

Optimize for low bandwidth
This option is only available for Enhanced Replication jobs.  When selected, Alike will utilize its differencing engine to only send changed data over the network.  This can allow you to perform replication jobs efficiently over a WAN or VPN.  If this option is disabled, Alike will bypass its differencing engine, which can will utilize more bandwidth, but can be faster on local networks.

Caveats
The ABDs used for ER jobs must be able to see all the required SRs.  This is most often an issue when you are replicating within the same Resource Pool, and Alike uses only one ABD.  In this case, the ABD must be able to simultaneous access the SRs for both the source and destination VMs.  For example, replicating from one XenServer's local storage to another XenServer's local storage in the same Resource Pool is not possible because the ABD cannot see both Xen hosts' local storage at the same time.


Enhanced Replication in Alike DR can be a powerful addition to your disaster recovery plans.  Its low cost, hardware and storage agnostic framework and powerful compression techniques allow it to protect environments both big and small.