There are a small number of caches that are safe to be configured to allow local node based storage in a clustered environment. That information is hidden in the code and some wiki documentation.

https://docs.moodle.org/25/en/Caching#Cache_definition_configuration

To allow administrator to have a simple view of whether a can is shared or not, it needs to be shown to them. I made original proposals in https://moodle.org/mod/forum/discuss.php?d=323628. This got some feedback and I've altered the implementation idea based on feedback from there.

To show which caches are relevant when completing mappings we have 2 changes;

1. Add a "Local cache" checkbox to each configured store.
2. Only show cache instances that are local when the cache definition can support local caches.

This allows administrators to choose the fastest possible storages and also decide if a particular cache is shared across the whole instance or is local per instance node.

The power to decide if a particular cache supports being node local is now in the hands of the developer who creates that cache. They are the ones with sufficient knowledge to know if it's been setup in a way that will work.