If you are running any version of the App-V 5 client or Sequencer prior to App-V 5.0 Service Pack 2 Hotfix 4 – stop reading. This does not apply to your environment. If you are running HF4 or sooner, you need to have a good understanding of the net effects of toggling VFS Mode on and/or off during package upgrade.
VFS Write Mode was introduced to (my words) “fix bad brake jobs” when it comes to application development. Applications that need to be able to read and write configurations files in normally protected directories or have modified ACL’s for standard users in order to write to program locations that only administrators would normally have access to (Notepad++ has a way to go back and forth between good and bad program configuration - http://blogs.technet.com/b/gladiatormsft/archive/2014/05/24/app-v-5-on-sequencing-using-tokenized-paths-pvad-s-vfs-and-vfs-write-mode.aspx.)
While VFS Write Mode involves setting a specific attribute within the package manifest, the effects on how the VFS (Virtual File System) and COW (Copy-on-Write) filter are handled for the user are significant. As a result, making any changes to this setting during package upgrade could have some effects.
Scenario 1: Package Upgrade where the VFS Write Mode setting is not changed
This one is pretty straight-forward. No considerations are needed. Nothing is changed in how the VFS handles things.
Scenario 2: Package Upgrade where VFS Write Mode setting is turned on when previously not enabled.
When this happens and directories previously existed in the COW but there was no “S” directory, then the original directory will be renamed to the “S” version and the permissions appropriately adjusted. The new “regular” directory will be created. So instead of just one directory (i.e. ProgramFilesX86) there will be two (adding ProgramFilesX86S.) For more information on the “S” directories, please refer to my previous post here (http://blogs.technet.com/b/gladiatormsft/archive/2014/08/29/app-v-5-on-the-now-sometimes-immutable-package-cache-location.aspx.)
Scenario 3: Package Upgrade where VFS Write Mode setting is turned off when previous enabled.
In this scenario, there will likely be existing two structures: directories appended with “S” and the regular directory with the relaxed permissions. When this happens, the relaxed permissions directories will be deleted and the “S” directories will be renamed back to their original names. If this is ever done unintentionally, you can imagine some of the issues that may happen so be careful doing this.
I would advise always avoiding Scenario 3. Scenario 2 will be quite common as many sequencers and packagers are now upgrading their pre-SP2 HF4 App-V packages in order to take advantage of the new VFS Write mode feature. The question I am getting asked a lot lately is whether it is better to re-sequence with VFS Write Mode on or to simply just perform a package upgrade. I would advise trying the package upgrade first to turn on the value. In most cases, this should work – but as always, I await your comments.