Kirx' Blog -

To Q: or not to Q: – that is the question | May 19, 2011

… at least that is one question that often is discussed in an App-V environment.

While monitoring an application setup with the App-V Sequencer, you will have to decide if you install the application to the C: drive (the installer’s default location) or to the Q: drive. Microsoft strongly recommends to always install to the Q: drive (an “MNT install”). However, many
applications proved to work fine after they were installed to C: (a “VFS install”).
This article discusses some of the myths and facts around that “C: vs. Q:” or “VFS vs. MNT” installations.

Note: To limit the size of that post, there is an “Extended Version” availabe as PDF at This PDF contains more detailed information, esp. the details to reason the table’s content.

[Update 2011-05-23: Tim Mangan blogged about an issue when Sequencing to Q: in the App-V 4.6 SP1 Sequencer.]

What’s up with the Q drive?

Microsoft App-V is the only application virtualization solution I know that incorporates the concept of a virtual drive letter. On the client, the App-V Client software creates a new (virtual) drive that can be considered as the “mount point” for all virtual applications. Whenever a virtual
application requests a file from the package, it will get this file finally from Q – even if the app requested the file from C.

(more details can be found in the PDF)

Each Virtual Package (like the OpenOffice Suite, certain Tools or Line-of-Business-Suites) has its own, dedicated “Primary Directory”
that is located directly under Q:. Each of these folders is individual per-package: two packages cannot share the same Primary Directory. The App-V Clients throws an error if two different packages try that. In the “ancient days” (actually before SP1 for App-V 4.6)) this Primary Directory had to follow the “8+3” naming convention, but that’s another story..

Install Directory and Primary Directory

During Sequencing, one or the other Sequencing Wizard Screen will ask you for the “Primary Directory”. This is where the Sequencer will collect all files that have been added or modified during Sequencing, also it will use this directory for its own, internal files (like osguard.cp) and folders (like SoftgridUserSettings).

Depending on the Sequencer version, the Screen in question will prompt for “The primary directory the application was/will be installed to”
before or after the actual monitoring.

The Primary Directory has to be on the Q: drive. There is no discussion about that.

The Install Directory, as referred to by this article, is the folder where you actually install the application to during Sequencing. You may be
prompted by the application’s MSI or Setup.exe or you may manually copy files to the Install Directory. Whether or not this will be a folder on the Q: or C: drive – this is what we’re discussing here. If you install an application to C:, every file later on will be copied into the “VFS” (Virtual File System) folder under the Primary Directory. This sometimes is called a “VFS install”.
If you install the application to a folder within the Primary Directory, this is going to be a “MNT install”, because the app is installed into the “Mount Point” (actually the Q: drive).

MNT vs. VFS: An Overview

Let’s start with a given statement: Microsoft strongly recommends installing applications to the Q: drive (hence perform an MNT install).

MNT install (Q:)

VFS install (C:)

Official Statement

Recommended by Microsoft Proven to be possible. Supported (but not liked) by Microsoft

Sequencing Performance

May allow faster Sequencing May slow down Sequencing

Execution Performance

May run faster on Client machines May result in slower applications

Installer Compatibility

Not possible with every installer Possible with every installer

General Execution Compatibility

Medium: Some applications may not allow to be run from a drive different to C: High: Most application allow to run from C:A very few applications will refuse to launch

Platform Compatibility (x86/x64)

High Medium / High (uncertain)

Application Update Compatibility

High High

Package Branching Support

Weak High

DSC Conflict Prevention

High Medium

VFS vs. MNT – Some Details

Please refer to the attached PDF for more details on

  • Official Statement
  • Sequencing Performance
  • Execution Performance
  • Installer Compatibility
  • Execution Compatibility
  • Platform Compatibility
  • Application Update Compatibility
  • Package Branching Support
  • DSC Conflict Prevention

Summary and Recommendations

As you can see, there is no easy answer to the question whether an application
should be installed to the C: or the Q: drive during Sequencing. At the end,
you will be required to try-end-error for some applications.

The recommendation is

  • If you intent to “branch” an application a lot, sequence it to  C:
  • Sequence Dynamic Suite Composition Components to Q:
  • Sequence all other applications to Q, except if
    • They cant’s be installed/run from Q:
    • You have lots of “sophisticated” Unattended Installations that point
      to C: (then Sequencing it might be much easier and faster)

[Update 2011-05-23: Tim Mangan blogged about an issue when Sequencing to Q: in the App-V 4.6 SP1 Sequencer.]

Details PDF

In the attached PDF you will find the content of this post plus

  • More details on the motivation
  • More details on the different aspects that have been examined for the summary (VFS vs. MNT details)

–> To-Q-or-not-to-Q_That-is-the-Question

Posted in App-V, Sequencer
Tags: , ,


  1. I’ve seen issues creating Package Accelerators when using a MNT install – using the same steps but with a VFS install then produces a successful accelerator.

    Comment by Aaron — May 19, 2011 @ 12:16

    • Great article, by the way ;)

      Comment by Aaron — May 19, 2011 @ 12:16

  2. there is an issue with vfs if you haven’t a c: drive

    Comment by cyber — June 30, 2011 @ 22:47

  3. in which cases we got for vfs? could anyone give me a clear description with example scenario, i would be very thankful for respone.

    Comment by Abhiram — December 10, 2012 @ 12:03

    • That’s not easy to answer, because there is that “performance loss threat” that Microsoft communicates. Some recommend to *always* perform an VFS install to C:\vapps\:

      – Because you don’t install to the Primary Directory (Q:\8.3) you don’t get that upgrade/save-as-new-version challenges
      – Because you do not install to ‘Program Files’ nor ‘Program Files (x86)’ you don’t get those potential x86/x64 issues I originally expected/decribed in the post
      – If you use ‘a unique value’ for (like the same string that you use for the PackageName or the PrimaryDirectory on Q), yo avaoid any potential DSC conflict on the file system level. If you use the ‘short name’ even the folder structure doesn’t get to long (some apps struggle with folder names > 256)

      Remember that you *always* should point to the Q:\8.3 folder when the *Sequencer* asks for the primary directory/root folder/asset directory.
      Of course this “VFS to somwhere completly different” only works if the application (installer) supports that. If an app can only be installed to C:\Programs\AppName, then you need to install it there
      Again, this is NOT a Microsoft Best Practice

      Comment by kirxblog — December 10, 2012 @ 23:44

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