[knot-dns-users] Key pre-generation

I have been looking into the key pre-generation capability of keymgr, and the following question has come up: Imagine I pre-generate, say, one month's worth of keys for a given zone. This zone is defined so that it will be signed automatically on bringing up the Knot server. Next I start the Knot server. What criteria are used in order to select the keys, among the pre-generated ones, to be used to sign this zone? The reason I am asking is because I pre-generated two years worth of keys for a particular zone, and when I started the Knot server it took a significant amount of time selecting the appropriate keys from among the pre-generated ones.