[166] in Athena_Backup_System
crippled mode aftermath and tape labels
daemon@ATHENA.MIT.EDU (Diane Delgado)
Sun Jan 7 13:37:31 1996
To: athena-backup@MIT.EDU
Date: Sun, 07 Jan 1996 13:37:21 EST
From: Diane Delgado <delgado@MIT.EDU>
This is in reference to Jonathon's question about what to do
with a tape which was labeled during crippled mode and how
to resolve the fact that its label information may conflict
with an external label already listed in the database.
After reading the "mtio" man page serveral times I
recommend that we do not attempt to re-write a tape label
on a tape which was written during crippled mode if it
has backup data on it. The man page seems to indicate
that while it is possible to write a record in a
non-append mode, that this could be a dangerous operation
on some devices:
>Care should be taken when overwriting
> records; the erase head is just forward of the write head
> and any following records will also be erased.
----------
Some possible suggestions to think about:
I believe it is probably easier to physically change the external
label of the tape if it conflicts with something already in the
database. After the external label is changed the tape could
be entered into the database either via the sql monitor or
we can add functionality to the master to support adding a tape
to a pool without labeling it. (As it
currently, exists the Master wants to label the tape at the time
it is added to the tape pool).
Conflicts with the internal id are probably less likely to occur since
our machines must run with synchronized clocks for Kerberos and the
internal id is simply a timestamp of when the tape was first entered
into the database. We probably have to deal with the 5 minute
skew which is allowed, since the possiblity of conflict would arise
there. We could ensure that labeling during crippled mode does
generate internal ids which are later than some known "safe time", where
the safe time might be the highest internal id value in the ascii db dump
or it might be a time after which the master was known to be down.
