Anyone ever seen PVS write cache filling up when the Optimize Drives (defrag) service is running?

Did you defrag you vDisk before read-only?

It's showing 0% fragmented, so I guess it was done...

OK, mine is disabled in our optimization GPO for our W16 build.

Take it you don't do FSLogix compaction?

If you are running anything that writes to the c:\ after booting the targets in read-only, you are going to have a bad time. We also disabled defrag with BISF + Citrix Optimizer

I would normally disable it too but it needs to be running for the compaction to work, unless I'm missing something (highly possible)?

Indeed I don't do the compaction for FSL, I'm doing it from the file server using Jim Moyle script.

is the Scheduled Task \Microsoft\Windows\Defrag --> ScheduledDefrag disabled? It should be. Then no scheduled disk defragmentation should happen. FSL will start the service when user logs out. (Startup type should be manual)

Will check