mountdtab - Table of local file systems mounted by remote NFS clients
The mountdtab file resides in the /var/adm directory and contains a list of all remote hosts that have mounted local file systems using the NFS protocols. Whenever a client performs a remote mount, the server machine's mount daemon makes an entry in the server machine's mountdtab file. The umount command instructs the server's mount daemon to remove the entry. The umount -b command broadcasts to all servers and informs them that they should remove all entries from mountdtab created by the sender of the broadcast message. By placing an umount -b command in a system startup file, mountdtab tables on NFS servers can be purged of entries made by a crashed client, who, upon rebooting, did not remount the same file systems that it had before the system crashed. Tru64 UNIX systems automatically call umount -b at system startup
The format for entries in the mountdtab file is as follows: hostname:directory Rather than rewrite the mountdtab file on each umount request, the mount daemon comments out unmounted entries by placing a number sign (#) in the first character position of the appropriate line. The mount daemon rewrites the entire file, without commented out entries, no more frequently than every 30 minutes. The frequency depends on the occurrence of umount requests.
The mountdtab table is used only to preserve information between crashes and is read only by the mountd daemon when it starts up. The mountd daemon keeps an in-core table, which it uses to handle requests from programs like showmount and shutdown.
Although the mountdtab table is close to the truth, it may contain erroneous information if NFS client machines fail to execute a umount -a command when they reboot.
mount(8), umount(8), mountd(8), showmount(8), shutdown(8) delim off