A mirrored repository has a writable copy on one server, often called the master server for that repository, and read-only copies on one or more servers, similarly called slave servers . The slave repositories are updated near instantaneously, subject to network speed, when the master repository is updated.
The primary purpose of most mirroring is to make read access (over the network) faster, by providing a copy on a nearby server for fetches and clones. One might, therefore, expect a single server to be designated as the master and several others as slave servers.
However, Gitolite mirroring is defined at the repository level, not for the entire server as a whole. For each repository, the administrator decides which server holds the master (writable) copy, and which of the other servers, if any, should make slave copies available to their local users. (This is why we speak of master repository and slave repository, not master server...