There are no official answers on this subject, but some questions in the forums led me to tap out some loving suggestions.
Of the six folders that FCP creates on a Scratch Disk Location, I believe three should always be kept local:
- Waveform Cache Files
- Thumbnail Cache Files
- Autosave Vault
...simply because the files within are usually so small that they gum up the performance of the SAN if you send them through the fibre.
That said, there may be others that tout the virtues of directing the Autosave Vault to the SAN, since this may be the only way that one could recover a project in case an editing bay goes down hard. I'm not so crazy about the idea, since again, we are populating both the storage and fibre net with small file traffic.
Project files: again, no set formula, and there are many superstitions out there, but it comes down to facility culture. Do different creatives need access to the project files, does the access need to be simultaneous? If those two answers are "no" then I see more sanity in working with the project file locally, but then copying the file back to the SAN when finished. Other facilities need simultaneous access, perhaps read-only access, to project files, and therefore opening and saving off the SAN are important.
Permissions issues of course factor in, and the savvy admin needs to do some thinking about to handle the folder(s) in which projects are saved. For example, creating a read/write ACL for all SAN members seems like a good idea for universal access, but also opens the door for someone writing over someone else's hard work without proper communication.
Lastly, again remember that saving projects to the SAN loads your fibre network with small file traffic. If at least you can create an affinity for the folder(s) that hold project files, that map to a separate storage pool than the "main" media storage pool, the performance of the SAN will be better overall.