Product Documentation

Merging VHD Differencing Disks

Sep 28, 2016

Merging VHD differencing disk files can save disk space and increase performance, depending on the merge method selected.

Citrix recommends merging vDisk versions to either a new base image or to a consolidated differencing disk. Each time the vDisk is versioned five times.
Merge methods include:
Note: A merge can only occur when no Maintenance version exists for this vDisk or when the vDisk is in Private Image mode. A merge starts from the top of the chain down to a base disk. A starting disk cannot be specified for the merge.

Merging to a New Base Image

A full merge to a new base image combines a chain of differencing disks and base image disks into a new single base disk. This new disk is the next version in the chain, which is given the file extension of .VHD. This method allows for the fastest disk access to the base image and is recommended when performance is more important than disk space (a new base disk is created for every merge performed).

Merging to a Consolidated Differencing Disk

A partial merge combines a chain of VHD differencing disks up to, but not including, the base disk into a new differencing disk. The new differencing disk has the same parent base disk image and is given the extension .aVHD. This method consumes less disk space than the full merge and the merge process is quicker than performing a full merge.

An automatic consolidation of differencing disks can be configured from the Farm Properties dialog's vDisk Version tab. On this tab, a maximum vDisk number is selected. When that number is reached, a merge is automatically performed and the availability of that vDisk depends on the mode selected on the tab (Production, Maintenance, or Test).
Note: A consolidated differencing disk merge is recommended when disk storage is limited or when the bandwidth between remote locations is limited, which makes copying large images impractical.

Merging Differencing Disks

  1. Right-click on a vDisk in the Console, then select the Versions menu option. The vDisk Versions dialog appears.
  2. Click the Merge button. The Merge dialog appears.
  3. Select to perform a Merged Updates or Merged Base merge.
    • To merge all differencing disks to a single differencing disk (not to the base disk image), select the Merged Updates option.
    • To fully merge all differencing disks into a new base disk, select the Merged Base option.
  4. Select the access mode (Production, Maintenance, or Test) for this version after the merge completes. If no access type is selected, the vDisk mode setting selected for automatic merge, on the Farm Properties' vDisk Version tab, is selected by default.
  5. Click OK to begin the merge process.

The time it takes to complete the merge process varies based on the merge method selected and the number of differencing disks to merge. After the merge successfully completes, the new version displays in the vDisk Versions dialog. The Type column displays either Merge Base if a full merge was selected, or Merge if a partial merge was selected.