Using Changed Block Tracking (CBT) for XenServer

Categories: CBT, XenServer, Backup, Replication, Alike v4


Alike (v4.5 and up) supports Changed Block Tracking (CBT) for Citrix XenServer 6.5 and above. This option can significantly improve job performance, and reduce the amount of I/O required to process them.

Please note: Alike’s CBT for XenServer is currently only available in Enhanced jobs for XS. Skip to more info about CBT Enhanced Replication.

For information on using CBT with Hyper-V 2016 and above, watch this short video.

Changed Block Tracking with Enhanced Backups

Because of its significant performance benefits, all customers who are able to use CBT are encouraged to do so. However, you should first become familiar with the requirements for this option.

Alike’s Changed Block Tracking for XenServer will maintain a single (reserved) snapshot on your VM at all times. This is necessary for the delta detection process. If the reserved snapshot is removed, CBT will be disabled until Alike can recreate it (on the next run of the job). You must have sufficient storage available in your XenServer SR(s) to be able to retain this extra snapshot in order to utilize CBT.

Note: At this time, drive exclusion is not available for CBT enabled jobs. This may change in a future release, but as of v4.5 CBT enabled jobs will protect all drives associated with selected VMs.

To enable CBT for Enhanced backup jobs for XenServer, simply select the checkbox in the job configuration as shown below. You will notice that CBT is simply an option for Enhanced jobs, and it is unavailble for Q-Hybrid based jobs. Furthermore it cannot be used in conjunction with the ‘fallback to Enhanced’ option.

alike 4.5

Once selected and started, this backup job will perform a regular Enhanced backup within Alike. However, on subsequent runs, Alike will leverage its CBT engine to determine the changed data from the previous backup. If a backup job ever encounters an error, Alike will need to recreate its CBT information again on the next run before it can leverage CBT again.

Changed Block Tracking with Enhanced Replication

CBT enabled replication works a bit differently than it does with Enhanced Backups. Additionally, replication with CBT works completely differently than traditional Enhanced Replication. Because of this, CBT Enhanced Replication is incompatible with regular Enhanced Replication jobs, and will always work with its own Replica target. CBT replication jobs will maintain copies of your VMs with the suffix “-AlikeDR Replica,” while traditional replication jobs use the suffix “-Alike Replica.”

Configuring CBT ER is extremely simple. Just check the proper box in the job configuration as shown below.

alike 4.5

CBT based ER is very different from traditional ABD based ER:

  • Uses no ABDs on either side of the replication
  • Is incompatible with Single Pass Backup & Replicate
  • Must perform a full export/import of the VM one time for the initial baseline/seed
  • Requires sufficient (temporary) disk space on your ADS for the initial export process
  • The target VM must maintain a (reserved) snapshot at all times
  • The target VM may be used, DR tested, and changed as much as you like between runs—Alike will fully revert the target system on all CBT runs

After the initial ‘seed’ replication is complete, the subsequent replication jobs will only need to transfer a fraction of the initial amount (the unique changed data between runs).

Final Notes

Please note: If for any reason (due to job failures, or other circumstances) Alike loses track of the state of either the source or target VMs, Alike will trigger another ‘baseline’ replication. Because this takes significantly more time, it is to be avoided whenever possible.

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