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App Layering and file writes ?



I can't seem to find an answer to this question so thought I'd ask in the forums.

In a basic App Layering / MCS non-persistent deployment where do file writes go ?


As in...

App Layering images with Elastic Layers & User layers disabled (no User Layer, no persistence for writes)

MCS Catalog for pooled, non-persistent VDIs


Where does the unidesk filesystem filter store file writes when a file is updated anywhere in the system given there is no cache or additional disks connected ?


I understand at reboot the image will revert back to its pre-booted file versions but during operations where are changes being stored?


Also does the location depend on the underlying hypervisor ?


My scenario is

Windows 10 VDI

Microsoft Azure 

Citrix Cloud

MCS Catalog, pooled using Azure Premium SSD with no write cache configured !


Thanks in advance ?



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OK thanks for the reply, so any file writes go into the image as if it were a normal VHD. Including OS updates, AV definition updates etc.. until the VM reboots and resets to pre-boot image I assume ?


So what happens when the filter drivers are involved (I assume when you enable "Elastic Layer", User Layers) ?


File writes go into the User Layer (filtered via the "AlwaysOnBoot" registry key)?


What about any writes outside of the User Layer (e.g. a path in the AlwaysOnBoot regkey) or a path in an Elastic Layer path ?


Also if you employ some sort of write-persistent caching or differencing technology (e.g. PVS cache, MCSIO or VMware differencing disks) where are the writes then? 

Do they go into the cache as normal then following the App Layering filter driver rules once committed to disk ?


I suppose in essence what I'm asking is when you deploy an image with or without User Layers enabled there is no need to deploy any other write enabling disk/caching technology to enable writes (outside of the configured User Layer areas) as the filter driver handles these in the normal way as a VHD write would be (till the image is rebooted) ?

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If you use the user layer its all writes except for a very limited number of folders.  User layer doesn't mean profile or user writes the user layer is for almost all writes so that you can do things like install applications that have services.


So for normal MCS it depends on whether you use MCSIO or not where the writes go.  For pooled desktops they go into delta files.  For persistent desktops they go into the boot disk.  For MCSIO based VDAs they go into the cache disk.


If you are using elastic layering but not user layers then a partition is added to the image where writes go but the above settings for MCS can change that as the writes will actually go to the place MCS is putting them.

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