Thank you for your post.
I don't know about a formula but you can find a suggested method of keeping an eye and deciding when to increase cache size in Knowledge Base Article #5874. See step 9 under "Optimize the Host Computer".
I hope this helps!
Not one to go against TSPigeon; but there are have been numerous instances where following the FMI rule to set the cache to the highest possible value does actually hurt stability (not performance).
From where I sit; the old best practice still stands: assign the cache high enough to hit 100% cache hits consistently but don't set it too much higher. It seems to make FMS14 more stable.
If your databases total 2 GB and your server has 16 GB RAM and you set your cache to 4 GB, which is only half of the 8 GB you could assign to it, common sense says that you will have 100% cache hit all the time and all your data will end up being cached.
Is this ideal ? IMHO, no. Especially if you have a SSD drive.
Cache is still faster than SSD drives.
I agree with wimdecorte and I use the method he suggested 98% of the time.
I have done deployments where performance improved with the entire file in the cache. This was done for an image management system that needed to display numerous images on the layout. Even with only using thumbnails the performance improved with a Cache large enough to fit the whole file in. The cache hit is usually at 100%, but the % Cache Unsaved also sits at near 0% all the time. Using scripts to commit and flush cache regularly seems to help. The user performance results were about the same with setting the cache to 80% of the file size so I later reduced it as much as possible while keeping the performance desired. That database is about 90% read and 10% write so this can also make a difference.
I am not sure if progressive backups help keep the data stored on the disk up to date but this would seem to make sense.
Thanks very much for the suggestions. Very helpful.
Now that I think about it, Wim's suggestion sounds like something I may have heard at a Devcon under the hood session. Don't ask me which Devcon.