Download video: Uploading Unreal Engine 4 (UE4) Built from Source to a Perforce Server
Channel: Michael Allar
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Blog post: http://allarsblog.com/2014/09/27/Setup-Perforce-Digital-Part2/
If you know what you are doing, basically, all .dlls and .exes should be writable, .pdbs aren't needed as they take up massive amounts of space, and DerivedDataCache/Saved/Intermediate/Project files are not needed as they will always be regenerated.