Categories: Backup, Replication, CBT, Alike DR, XenServer, Hyper-V, Alike v3, Alike v4

Introduction

Alike DR Edition includes a powerful business continuity feature called Enhanced Replication (ER). ER is able to efficiently 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.

Enhanced Replication can replicate VMs regardless of the differences in source and destination storage, hypervisor type or version or edition.* Both Citrix XenServer and Microsoft’s Hyper-V are supported for Enhanced Replication.

Please Note: When replicating from XenServer to a Hyper-V host, only Q-Hybrid technology is supported at this time. As of v4.5, Alike supports CBT for backup and replication jobs; however, CBT ER is not compatible with regular Enhanced Replication jobs. Learn more about CBT and Enhanced Replication.

This article will provide an overview of Enhanced Replications features and configurations.

Enhanced Replication Overview

Enhanced Replication allows you to copy a point-in-time snapshot of a VM from one system or SR to another. This keeps the target VM up-to-date with the source VMs 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 off of shared storage. With ER, you can schedule a replication of your VMs on a nightly basis to you host servers’ local storage(s).

In the event of a SAN or SR failure, you would be able to boot the VMs off of your hosts’ local storage for a rapid service recovery. Once the production SR is back online, ER allows you to configure replication in reverse, sending your VMs back to production.

As mentioned, ER jobs only the differential data to lower the amount of bandwidth required when replicating a VM over your network.

Because of these capabilities, using Enhanced Replication for DR purposes is very common. Replicating VMs from a primary pool or location to an offsite DR pool can provide a significant amount of site level protection. And thanks to Alike’s deduplication, ER can efficiently replicate even over lower bandwidth connections.

Configuring ER

Enhanced Replication provides the option to use either Quadric Software’s ABD (Alike Backup Delegate) technology, or Alike’s unique Q-Hybrid technology (Please note: it is not supported to switch from one replication technology to another between replication runs).

When choosing between ABD and Q-Hybrid for replication, it is important to keep the following rules in mind:

• An ABD is always required for the destination, if the destination is XenServer.
• When replicating from XenServer to Hyper-V, Q-Hybrid is required.

To use an ABD based ER (Xen only), an ABD template image must be deployed to either the source or destination (or both) Resource Pool, and at least 1 IP address must be give to each.  To learn more about ABDs and how to deploy them, refer to our KB article, “Understanding ABDs.”

If your source and destination VMs are both in the same Resource Pool, Alike may only require one ABD, but often it will require two. See the caveats section below for more details.

Once the ABD Images are deployed, configuring ER can be done when creating a new Replication job in the Alike Web UI

1. Go to Tools–>Settings–>Storage Settings
2. Select your destination host and storage location
3. Go to Jobs–>New Job–>Replication
4. Choose the replication method (Q-Hybrid or Enhanced) and job options.
5. Click on the image below to see a demonstration

Enhanced Replication Options for XenServer & Hyper-V

When a XenServer destination host is selected in the replication job, you are provided the options to use XVA, ABD or Q-Hybrid methods. If a Hyper-V host is selected as the target, you may enter a custom VHD path, or leave the default location.

Replication Method

When selecting the replication technology for a new job, remember that XVA and ABD methods are supported only in XenServer environments. Q-Hybrid will work for both XenServer and Hyper-V. Keep in mind that CBT replication jobs are not compatible with regular Enhanced Replication jobs.

Both ABD and Q-Hybrid are high performance, and will transfer only the data that has changed between replication jobs. XVA based jobs will transfer the entire VM data every run, and will also require temporary space in your ADS to function.

Enable Replication Cache / 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.  And this option is ignored (not available) for pull replication jobs.

Single 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 one job for both features.

Please Note: Single pass isn’t available for CBT replication jobs.

Replication Data Flow Direction (ABD Jobs Only)

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 as follows: 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.

Final Notes

Caveats for Enhanced Replication:

• 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.
• 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 both large and small environments.