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Document ID: 5123
Subject: Why do backing up VHD files take long
Creation date: 6/7/17 9:02 AM
Last modified on: 12/14/18 11:41 AM

Why do VHD files take long to backup?

Users often complain backing up VHD (Virtual hard drives) files take a very long time in Syncrify. This page explains the reason and also an alternative method to use.

The Rsync algorithm used in Syncrify tries to determine the delta between two versions and then sends this delta over to the destination, which is then merged with the older copy of the file on the server. This is called the block-matching process.

The amount of time spent in determining delta (block-matching) is directly proportional to the amount of data that is modified within a file. For example, if a file is modified towards the end, matching occurs very quickly. A good example of such a file is a PST file for MS Exchange.

On the other side of the spectrum, a file that is modified from all over will not only take a long time for block matching to occur but the resulting delta will also be almost as big as the actual file. VHD files are notorious when it comes to changes. Typically, a hard-disk gets modified from blocks that physically reside in different locations and therefore, it is very likely to have a VHD file that is modified about 90%.

In summary, there are two problems when using Rsync with VHD files:

  1. It takes a long time to match blocks
  2. The resulting delta is often as big as the actual file, diminishing the network bandwidth benefit you achieve when backing up other files.
Run the file through Block-Diff Wizard to see if you will gain from using rsync on a VHD file.

An alternative method

If Syncrify client and server are taking to each other on a fast network, it may be faster to just use Simple Copy, which will copy the entire file without matching any blocks.

User comments

Posted by Rick Cassel on 10/19/21 10:24 AM

Maybe its time for a new algorithm, Microsoft Backup doesn't seem to have a problem with it. Veeam, Altaro, Nakivo, Acronis, Unitrends, Paragon have it figured out, I can visualize a VHD checkbox in Syncrify that could call for a VSS Snapshot and use that to do the comparing lighting fast, then slurp the snapshot across the wire and merge it lighting fast. Or call out to a modified Rsync alogrithm. I can't use this product with a tiny 30 GB VM. It took days.

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