Please avoid top posting on this list. Use interleaved posting instead as this simplifies following a conversation.
The high-availability plugin relies on a shared file system and consists of multiple Gerrit servers persisting git repositories in the same shared volume.
You can run it with one active and one or multiple passive servers or multiple active servers.
noteDB is not a separate storage system but stores Gerrit metadata in the respective git repository.
E.g. review comments added on patchset 1 of a change "refs/changes/01/801" for repository "foo" are stored in repository "foo"
and the custom ref "refs/changes/01/801/meta" points to this metadata so that Gerrit can look it up.
The actual patchsets which represent iterations of the change refs/changes/01/801 are referenced via additional refs:
refs/changes/01/801/1: patchset 1
refs/changes/01/801/2: patchset 2
...
-Matthias