I have a few fundamental points I'm unsure of and was hoping someone could help clear them up for me, I couldn't find an exact answer in the documentation. They're around the use of the Microsoft Gallery image vs using a custom image / VHD stored in a blob.
What reasons (other than to install applications and change settings) in the image itself, would justify the use of a custom image? Greater control over installation of updates?
If I use a custom image, and the WVD instances are built from this image, can those hosts make changes to the source image?
If i use a custom image, logged into it via WVD and ran windows update, would this write back to the source VHD image in blob storage?
If I use the Microsoft gallery image, is this read only? Do Microsoft keep this up to date? If they do, when are those updates applied to the image seen in the host? Is it live, or do the hosts need to be removed and rebuilt for this to take effect?
My understanding would be that it's kind of similar to a PVS streamed image. Hosts build using the 'source' image and remain as they were when they were built, until the host is taken down (or destroyed) and a new one provisioned. When it is re provisioned it uses the VHD image as it is right now. The host pools have no write access to the underlying image (or they're just running from a copy of it, taken at the time of provisioning?)
I've limited experience, but I'll try and answer as many of your points as possible:
1. Primarily the usage of a custom image is to aid in the configuration process. As you say programs, registry tweaks, user language/input options, required files and nay other tweaks can be performed upon one VM. Then it's shut down and a "Gold" image is created. You can then use this to create/spawn your pool of Vms from as a base image. It's particualry useful if you have a large WVD pool with many hosts. As one might imagine once you get over 5 or more VMs perfomring the required setup "Tweaks" can take a long time, with the possiblity of missing something along the way.
2. Any hosts built from a custom image are ONLY built from that image. So their OS disk is formed of a COPY of the custom image. No changes are made to the image in any way.
3. As stated before, the custom image is not 2logged into", it's better to think of it as the image used as a copy for each VMs OS disk. If you need to update a custom iamge you would need to create a Vm using it as the image, then make your required changes, then create a new image from that VM to replace the older one.
4. The Microsoft gallery image is maintained by Microsoft, so in that sense is pure. As for whether Microsoft keep it up to date, I'm unsure. A quick Google produced no relevant results. So I'll have to leave that to someone else to answer. One would hope the gallery images are kept somehwat up to date. I do know I've still had Windows updates to install when using a Microsoft Gallery image, so how up to date they are is debateable..