We have 1.5 TB database that's shown an error and block all commands. The error is : "ERROR: database is not accepting commands to avoid wraparound data loss in database "dbname" HINT: Stop the postmaster and use a standalone backend to vacuum that database. You might also need to commit or roll back old prepared transactions."
I tried to do vacuum in the backend mode. Also I tried to set zero_damaged_pages = on then do the vacuum again but same error appeared. The error appeared after two hours of vacuuming where verbose shows passing tables.
I don't see any way that error message can be generated while in standalone mode.
Are you sure you sure you don't have multiple instances running, and you are mixing up the logs between them?
