By default, Elastically Assigned Layers are loaded at the end of the login process so that the user can get to a desktop quickly and not have to wait for their Elastic Layers to surface first.  This feature is often called "late load" of elastic layers.  However, especially with Session Hosts, you may find that the Elastic Layers are not available quickly enough.  In a XenApp environm,ent, for instance, your first attempt to use an application which is being elastically delivered might result in a "file not found".  The second attempt (and all subsequent attempts) works fine, because once the layer is attached, it remains attached.

It is possible to make Elastically Assigned Layers attach at the beginning of the login process.  You can set this per layer (which applies to all images using the layer) by editing the properties of the layer in the Management Console and setting Compatibility Mode.  That forces the layer attachment to happen first, and not wait until the login is complete.

You may desire to set an image to do this for all layers, rather than having a layer do this for all images.  This is not in the Management Console, but is available as a registry key.

There is a registry key called MaxCompositingPhase that you need to set.  "Compositing" is the process of surfacing and using a layer disk, and there are two phases of the login process where it can be set.  Setting the MaxCompositingPhase can limit all layers to phase 0.  Otherwise, layers will use phase 0 or 1 depending on how they are configured in the Management Console.

If you set this to 0 (zero) this will effectively shut off Late Load. Setting it to 1 will allow other settings to be 0 or 1, and so on. By default this is set to allow layers to be composited before or after login.

[HKLM\SOFTWARE\Unidesk\ULayer]
"MaxCompositingPhase" (DWORD)

0: Only composite layers before login completes
1: Allow layers to be composited before or after login has completed

To force an image to disable late load for all elastic layers regardless of their normal setting, create and set this key to 0.  This can be done by booting your published image in a persistent/private mode and setting the key.  It needs to be present when unifltr.sys starts, so it will not take effect until the machine reboots after the registry key is set.  Therefore a GPO may not happen at the right time.

You shoudld also be able to create an App Layer which contains this key, or put it in the platform layer, and all image templates which include that layer should have this value set properly.