Apologies for the delayed response.
There are a number of cases where you will need memory even under full eviction. For example: when you modify/add a value, we need to keep it in memory until it is flushed to disk. If changes happen faster than we can persist to disk, we’ll run out of memory and won’t be able to take more changes. Rebalance or compaction also need to bring in keys and values into memory so they are also going to suffer if there aren’t enough free pages of memory that is available to use.
It is hard to say what the right value is but we may be able to give you some round number if you can answer a few questions;
avg key size, avg value size, total keys, total number of buckets, total number of indexes, xdcr enabled/not enabled, working set (active portion of your keys at any given time), HW per node and number of nodes.