st(4) and mt eom

[John Nemeth <jnemeth%cue.bc.ca@localhost>]

     If you issue an "mt eom" (forward to end of media), the driver
loses track of the tape position.  This seriously messes with
Bacula's tape handling.  Since Bacula expects the driver not to
lose the tape position I get the feeling there are other operating
systems that don't.  I found this code in st.c:st_space():

        error = scsipi_command(st->sc_periph, (void *)&cmd, sizeof(cmd), 0, 0,
            0, ST_SPC_TIME, NULL, flags);

        if (error == 0 && (st->flags & ST_POSUPDATED) == 0) {
                number = number - st->last_ctl_resid;
                if (what == SP_BLKS) {
                        if (st->blkno != -1)
                                st->blkno += number;
                } else if (what == SP_FILEMARKS) {
                        if (st->fileno != -1) {
                                st->fileno += number;
                                if (number > 0)
                                        st->blkno = 0;
                                else if (number < 0)
                                        st->blkno = -1;
                        }
                } else if (what == SP_EOM) {
                        /* This loses us relative position. */
                        st->fileno = st->blkno = -1;
                }
        }
        return error;
}

Notice the SP_EOM case.  Can any SCSI experts, in particular SCSI
tape experts, shed some light on this and what can be done about
it?

     I have found a workaround for Bacula which is to tell it about
this problem.  If you do that, Bacula will do "mt fsf 65535" (and
pray that there aren't more files then that on the tape).  The tape
I have with the largest number of files is at 1186, so this will
do for now.  Still, it would be nice to fix the underlying problem.

     For those wondering, my bacula-sd.conf contains:

Device {
  Name = LTO-4a
  Archive Device = /dev/nrst0
  Device Type = Tape
  Media Type = LTO-4
  AutoChanger = yes
  LabelMedia = no
  Drive Index = 0
  AlwaysOpen = yes;
  Removable Media = yes;
  Random Access = no;
  Maximum File Size = 2GB
  Automatic Mount = yes;               # when device opened, read it
  Spool Directory = /bacula/spool-sd
  Hardware End of Medium = No
  #Fast Forward Space File = No
  BSF at EOM = yes
  #
  # New alert command in Bacula 9.0.0
  #  Note: you must have the sg3_utils (rpms) or the
  #        sg3-utils (deb) installed on your system.
  #        and you must set the correct control device that
  #        corresponds to the Archive Device
#  Control Device = /dev/sg??  # must be SCSI ctl for /dev/nrst0
#  Alert Command = "/usr/pkg/libexec/bacula/tapealert %l"

  # Enable the Alert command only if you have the mtx package loaded
# Alert Command = "sh -c 'tapeinfo -f %c |grep TapeAlert|cat'"
# If you have smartctl, enable this, it has more info than tapeinfo
# Alert Command = "sh -c 'smartctl -H -l error %c'"
}