Loose Deduplication Caching Option

Categories: XenServer, Hyper-V, Physical Systems, Alike v4


Included in Alike v4.1 is a powerful option to help improve your initial backup performance, for certain environments. The ‘Loose dedup caching’ option allows Alike to use similar VMs as a basis for deduplication caching when backing up a VM for the first time.

This option is particularly powerful for environments with many similar, or nearly identical VMs, such as VDI (virtual desktops) or systems hosting XenApp, Terminal Servers, or any other systems cloned from a ‘golden image.’

Please Note: This option is intended for initial backups only. If you choose to enable it, please read this article in its entirety, and disable the option after your initial backup load is complete.

To understand the benefits of this feature (and when and why to enable it), you must first know how Alike backups work. When Alike begins a backup of a system, it will first check to see if there are already any fully committed, and validated backups of that system in its ADS.

If so, Alike will be able to use this previous version history in order to calculate a changed block list, which is used to greatly reduce your backup times.

This ‘dedupe caching’ allows Alike to avoid sending a significant amount of data through the source-side deduplication process, eliminating disk and network I/Os, and processing both at the source, on the Alike server itself. Overall, this process is a large part of why subsequent backups are so much faster than initial backups in Alike.

With the ‘loose dedupe caching’ option enabled, Alike is able to draw upon the version history of similar systems in order to provide this deduplication caching for an initial backup. This shortcut can drastically reduce the amount of time and processing required to obtain an initial set of backups in Alike.

Currently, Alike’s determination of similar system is quite simple. In order to be usable for this option, a similar VM must match the following criteria:

  • Have the exact same number of disks, of the exact same size
  • Have the same detected Operating System (as seen on Alike’s System Explorer page)
  • Have a committed and validated backup

Example usage:

Using this option is easy to set up. Simply enable it the feature under Settings->System Settings-Advanced Settings->Allow loose dedupe caching matching. Click here for a demonstration.





Then, perform a backup of your first VM, allowing Alike to completely commit and validate the backup (green checks under the VM’s backup list). Once that’s complete, simply backup the rest of your ‘similar’ VMs. You should see a line in your job log details that reads:

“No previous backup available for dedup caching, using similar VM (name) based on settings”

Once Alike has finished processing the initial backups for your VMs, it is recommended that you disable this option.

Final Notes

Here are some of the risks of using the “Loose Dedupe Cache” Option:

This option can provide dramatic improvements for your initial backup performance. However, leaving this option enabled over time, especially in complex environments, exposes a small risk of backup failure of newly discovered VMs. This problem could occur if Alike were to choose a ‘similar VM’ for a new, initial backup.

During the backup process of that new system, the ‘similar VM’ is deleted/purged from Alike’s datastore. While this situation is very unlikely, and risk of backup failure is undesirable, and should be avoided.

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