Hi, On linux, many filesystems default to remounting themselves read-only when metadata IO fails. I.e. one common reaction to disks failing is a previously read-write filesystem becoming read-only.
When e.g. trying to create a file on such a filesystem, errno is set to EROFS. Writing with pre-existing FDs seems to mostly generate EIO. In errcode_for_file_access(), we map EROFS to ERRCODE_INSUFFICIENT_PRIVILEGE. An error code that's used very widely for many other purposes. Because it is so widely used, just searching for log messages with an ERRCODE_INSUFFICIENT_PRIVILEGE sqlstate isn't promising, obviously stuff like ERROR: permission denied to set parameter \"%s\" isn't interesting. Nor is EROFS a question of insufficient privileges - the filesystem is read only, even root would not be permitted to write. I think ERRCODE_IO_ERROR would be more appropriate than ERRCODE_INSUFFICIENT_PRIVILEGE, but not exactly great. The only real downside would be a slightly odd sqlstate for postmaster's creation of a lock file. If a tablespace were mounted read-only, IO_ERROR actually seems fine. Greetings, Andres Freund
