@Majorheading(File Interchange and Removable Media)
Removable media represent a major coherence problem faced by Project
Athena.  The section first describes the problem, then offers several
different solutions, each of which has at least one serious defect.

@heading(The problem:)

Although they are almost identical, a UNIX file system created on an
RT is not directly readable by a VAX, and vice-versa.  The underlying
reason is that the two architectures store objects other than
character strings in different byte orders, and this difference is
reflected in the stored structures of the file system.  This problem
becomes apparent in two cases in which the potential exists for one
machine to read a file system created by the other:

@begin(enumerate)

Although 1.2 Megabyte diskettes used by the two kinds of
machines are physically identical and are soft-formatted with
identical sector layouts, a file system written on one cannot be read
on the other.

A file system that is created by one of the two kinds of machines on a
remote virtual disk cannot be read by the other kind.  Although not
strictly a "removable medium," there are enough analogies between
remote disk and removable media that it is useful to discuss them
together.


@end(enumerate)
Student users are affected by this problem in several ways.

@enumerate{

For each locker and each diskette, a student must choose one class of
machine on which that locker or diskette will be used.  If the choice
is made based on the availability of a machine in a living group,
that may be the wrong choice for the most appropriate workstation
cluster used by a class in which that student enrolls.  It may also
not work in an electronic class or seminar room that has machines of
the other architecture.

System and class libraries must be provided in two copies, once in
each format.  Since system libraries contain primarily binary
programs, which are not interchangeable between the two architectures,
there must be two copies of those libraries in any case, and the lack
of interchange is not a serious limitation.  The same thing is true
for some, but not all, class libraries.  Maintaining two copies of a
library is an additional burden when the library contains mostly text
material.

}

@heading(Solutions)

@subheading<Solution A.  Tell students to use separate diskettes for
the two kinds of machines.>

Advantages. . .
@enumerate{
Procedure is simple to describe and understand.

No development effort required.
}
Disadvantages. . .
@enumerate{
This solution ducks the problem, rather than solving it.

To move files from one kind of file system to another one must find
two free workstations of different architectures, log in to both,
mount the two file systems, and copy files back and forth.

}

@B<Solution B.  Arrange that every public workstation cluster,
living group, and class or seminar room have a 50-50 mix of the two
kinds of workstations, and tell students to always use the same kind
of workstation.>

Advantages. . .
@enumerate{
No development effort required.

Procedure is simple to describe and understand.
}

Disadvantages. . .
@enumerate{
This solution ducks the problem, rather than solving it.

Requires replanning present clusters.

Breakage: when students don't happen to split evenly, there
will be a shortage of one kind of workstation; the numbers deployed
in each cluster would have to be increased slightly to compensate.

Some application programs run only on one kind of workstation,
leaving students who have diskettes of the other format out in the
cold.

}

@subheading<Solution C.  Use a PC/DOS-format file system on diskettes>

Advantages. . .

@Enumerate{
Permits complete interchange of text files between RT and VS-2.

Interchange with AT-class PC-compatibles becomes available.

Format is very robust--if user changes diskettes without informing
the operating system, its contents probably won't be damaged.

Code is available off-the-shelf with RT PC.
}

Disadvantages. . .
@enumerate{
Information stored on a diskette is not part of a mounted
file system, so files must be explicitly moved from the diskette to
the fixed disk of the workstation before use.

The DOS file system format limits file names to 8 upper-case
characters plus a three-character name-extension.

Permission is needed to use the RT PC code on the VAX.

May require development of a file system salvage and repair program
that runs under UNIX.  (A version of CHKDSK.COM for the PC.)

}

@subheading<Solution D.  Modify the DOS file system code that came
with the RT PC to allow it to handle UNIX standard file names.>

Advantages (as compared with solution C). . .
@enumerate{
The name of a file on the diskette can be identical to the
name it has when it is moved onto the workstation.
}

Disadvantages as compared with solution C. . .
@enumerate{
Probably requires substantial development effort to allow
variable-length names, and substantial checkout effort to verify that
robustness is still maintained.

Ability to exchange files with AT-class machines is lost.
}

@subheading<Solution E.  Develop a program that does in-place conversion of an
unmounted RT-format file system into a VAX-format file system and
vice-versa.  The UNIX mount command can be modified to detect
attempts to mount a file system that has the wrong format and suggest
that the user run the conversion program.>

Advantages. . .
@enumerate{
The resulting file system can be mounted in the storage
hierarchy of the workstation and files on the diskette
can be read and written directly by application programs.  Complete
exchange of text files is possible.

}

Disadvantages. . .
@enumerate{
The resulting file system is quite fragile; the user must be
careful to shut down and "umount" the file system before removing a
diskette.

The available tools for file system repair require a high
level of wizardry.

Significant development effort is required.

Start-up overhead: all directory blocks of a file system
must be rewritten before the file system can be used.

If a failure occurs during conversion, the resulting file
system is probably irretreivably damaged.

}

@subheading<Solution F.  Modify the UNIX file system of Ultrix and
4.2A to allow disk-stored file systems that are of either format.>

Advantages. . .
@enumerate{
The resulting file system can be mounted in the storage
hierarchy of the workstation diskette
can be read and written directly by application programs.  Complete
exchange of text files is possible.

Applies to RVD lockers as well as diskettes.
}

Disadvantages. . .
@enumerate{
Major development effort, and has to be integrated into two
different kernels.
 
The Athena kernel becomes significantly different from the
standard UNIX kernels of the two vendors, leaving Athena with a
long-term maintenance problem unless both vendors can be convinced to
pick up this change.

The resulting file system is quite fragile; the user must be
careful to shut down and "umount" the file system before removing the
diskette.  Similarly, if network problems cut off communication to an
RVD server, the risk of damage to the stored file system is high.

The available tools for file system repair require a high
level of wizardry.
}

@subheading<Solution G.  Abandon removable media and use a
network-based remote file system instead.>

Advantages. . .

@enumerate{
The resulting file system can be mounted in the storage
hierarchy of the workstation and files on the remote file system
can be read and written directly by application programs.  Complete
exchange of text files is possible.

Minimal implementation effort, mostly in administrative
support.

Robustness is very high--the central file system is
operated by staff members.

Relatively easy to document and explain.
}

Disadvantages. . .

@enumerate{

Cost of owning and operating central storage for all student files is
prohibitive.

Fails to meet detachability requirement.  A student cannot get at
private files from a private workstation located in an apartment.

Performance of available remote file systems for large-scale
use in student lockers is questionable.

Requires kernel changes to both Ultrix and 4.2A.  (NFS may
become available with Ultrix 2.0)

Leading candidates (NFS and Vice) have licensing problems.
}

@comment[end of @Majorheading(File Interchange and Removable Media)]