Enhanced Replication with Alike DR
Alike DR Edition includes a powerful 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 and performance. ER will keep a remote VM synchronized with its source on a schedule. Furthermore, ER will only send the changed data between runs, thereby saving time and bandwidth. 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, using the 'overwrite target' (default) 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 can 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 allows you to configure replication in reverse, sending your VMs back to production.
ER jobs only the differential data, to lower the amount of bandwidth required when replicating a VM over your 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 deduplication, 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 Image must be deployed to both the source and destination Resource Pool, and at least 1 IP address must be give to each. 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 may only require one ABD, but often it will require 2. Please see the caveats section below for more details.Once the ABD Images 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.
Enhanced Replication Options
Synchronize 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.
Trust destination integrity
Every time Alike performs an ER job for a particular VM, it will cache some deduplication information for the target VM. This allows Alike to speed up the ER jobs, especially for larger VMs. However, if the target VM changes (is powered on or is modified) between backup jobs, this setting can be dangerous for the target VM's integrity. Please note, Alike attempts to detect if the remote VM has been modified between runs, and will recreate its cached data if required even if this option is selected. Also note, this option is ignored (not available) for pull replication jobs.
One pass Backup and Replicate
When this option is selected, Alike will record a backup version for each VM that is replicated during the replication process. The backup and replication are performed on the same pass, and have the exact same data version. This can save a tremendous amount of time by allowing you to schedule 1 job for both features.
Replication data flow direction
Alike is able to alter the direction your replication jobs flow to suit your particular requirements. Which direction to chose is up to you, but the rule of thumb is: If the source VM is closer to the ADS, use push replication. If the target VM is closer to the ADS, use pull. Pull replication jobs can be very useful to replicate a VM running in a remote location to a local XenServer/Pool. Push replication is best for sending VMs to an offsite location. The default is push, which is recommended in most cases.
Caveats
The ABDs used for ER jobs must be able to see all the required SRs. Alike will attempt to spawn ABDs in the best location to perform ER jobs, but your source/target XenServers in an ER job must be able to see the shared SR where your ABD image is located.
N.B.
Targets of ER jobs DO NOT REQUIRE A LICENSE. You may deploy an ABD image to an unlicensed host.
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.
