"Witness" comes from Frugal Paxos [1] (AFAIK, and not cited in the Megastore paper directly from what I saw while skimming it a few minutes ago) and indeed means an acceptor who does not contain a state machine replica, but does store the log and participate in elections.
"Observer" is not as well specified of a term [2], but from what I can find observer means non-voting replica which stores the log and a state machine replica.
Both of these make sense depending on the goals of the system. Observers make it easier to add new replicas without changing the size of the quorum, and witnesses make it cheaper to increase fault tolerance.
I'm not as familiar with the literature on replication for file systems as I am with state machine replication, so perhaps the usage of those terms have diverged since then. Regardless, I think my analysis is correct for state machine replication. Thanks for the link!
"Observer" is not as well specified of a term [2], but from what I can find observer means non-voting replica which stores the log and a state machine replica.
Both of these make sense depending on the goals of the system. Observers make it easier to add new replicas without changing the size of the quorum, and witnesses make it cheaper to increase fault tolerance.
[1]: https://lamport.azurewebsites.net/pubs/web-dsn-submission.pd... [2]: https://cse.buffalo.edu/tech-reports/2016-02.orig.pdf