[ih] Distributed file systems [was: As Flag Day approaches at CMU]

Sat Sep 6 16:24:30 PDT 2025

I doubt anything was published.   It just wasn't that big a deal, and 
wasn't part of the research mission.

The "distributed file system" I described was "a hack" probably coded 
overnight when someone realized that it could be easily done by using 
the mechanisms that already existed inside ITS - like the JOB/BOJ 
pseudo-devices.  Such work wasn't part of any official project, but was 
done primarily to mitigate the scarcity of CPU cycles, memory, or disk 
space.   It might have been mentioned in the annual report of the AI Lab 
work, but I doubt there was ever a paper published.

With several ITSes on the ARPANET, and the hassle of dealing with FTP to 
move files, someone likely noticed that a "remote disk" capability would 
be useful and was an easy thing to try.   I recall someone once 
wondering why there was a lot of traffic between hosts on the MIT IMP, 
where other sites traffic patterns were mostly long distance.  MIT used 
the IMP as a poor man's LAN, and made it as easy as possible by coding 
up things like remote disks.

If anyone else had "done it first" I never heard about it.  There was no 
network yet, so information about other projects was not readily 
available.   Professors might have read journals, but hackers mostly 
wrote code.

Most of the OS changes to ITS were done by the MIT AI Lab.   But their 
focus was AI, and changes to the OS were often done to help with some AI 
project.  In DM, we changed software as needed, mostly focussed on 
research on use of the network such as email.

AI changed their PDP-10 hardware when that was useful, e.g., by adding a 
new instruction to ROTate memory in a counterclockwise direction, which 
was helpful to the AI Chess program.   I recall someone at some point 
made some hardware changes (might have been on the DM machine) that 
enabled a program  to be run "in reverse" for a bit.  That was helpful 
in debugging to figure out how the OS code actually got to some weird 
place, e.g., in some data structure, where it crashed because data as 
"instructions" made little sense.

ITS was a lot like Unix, in the sense that it was not an official 
project to research issues of operating systems.  That was more Multics 
territory.  ITS was just a tool to be used and modified as needed to 
help with the actual research topics of AI, DM, and later ML (MathLab) 
and MC (Macsyma Consortium).

The DEC field service techs used to hate coming to ITS land, but also 
liked it because they always learned something.

Jack

On 9/6/25 14:27, Brian E Carpenter wrote:
> I've never looked into the early history of distributed file systems. 
> Was that work at MIT ever published? Was it pioneering or did someone 
> else do it first?
>
> My favourite paper in that area is the "Unix United" paper [1] from 1982.
>
> [1] https://doi.org/10.1002/spe.4380121206 (paywalled) or
> http://homepages.cs.ncl.ac.uk/brian.randell/Papers-Articles/399.pdf
> Regards/Ngā mihi
>    Brian Carpenter
>
> On 07-Sep-25 08:04, Guy Almes via Internet-history wrote:
>> Jack,
>>     Thanks very much.
>>     So this was in place by the mid-70s, right?
>>     -- Guy
>>
>> On 9/6/25 3:15 PM, Jack Haverty via Internet-history wrote:
>>> ITS at MIT circa early 1970s used a naming convention for files --
>>> <device>:<directory>;<name1> <name2> So, for example, I logged in to
>>> MIT-DM as JFH.  My files on disk were things like DSK:JFH;THESIS TJ6
>>> File names were limited to alphanumerics of 6 characters or less
>>> (motivated by what you could encode into a 36-bit PDP-10 memory 
>>> location).
>>>
>>> Once the ARPANET and NCPs appeared, the 'net was a new toy, so 
>>> people of
>>> course experimented with how to use it.  I don't remember the 
>>> details or
>>> timing (sometime in early 1970s), but at but at some point the Message
>>> Of The Day announced a new capability - you could use files on some
>>> other ITS machine just by using a different <device> to specify the DSK
>>> on some other ITS machine.
>>>
>>> So, for example, from the MIT-AI machine a user could get to my file on
>>> the DM machine by specifying DM:JFH;THESIS TJ6.
>>>
>>> Similarly, from my account on MIT-DM, I could get to another machine's
>>> files by using a name such as AI:TK;NEWS ITS to get at Tom Knight's 
>>> file
>>> on the AI machine.
>>>
>>> This provided more flexibility than FTP.  You could use a remote 
>>> file in
>>> any program that knew how to use files on devices.  To the program, the
>>> remote disk looked and behaved like a local disk. (More or less -
>>> problems of "global LANs" were still be be surfaced)
>>>
>>> I don't recall at all how this worked, or who implemented it. IIRC, it
>>> took advantage of an interprocess communication capability called the
>>> "JOB/BOJ device", which enabled one program to open a JOB device, and
>>> another program to open the corresponding BOJ (JOB reversed) device, 
>>> and
>>> send whatever they liked back and forth.  But I don't remember details.
>>>
>>> We also had the ability for one process (aka "job") to map some or all
>>> of another process' address space into its own address space. I can't
>>> recall if anyone got motivated to get that working across the ARPANET
>>> though.   If so, it would probably have been done using the same
>>> internal mechanisms that got the remote file systems capability.
>>>
>>> However, for anyone curious, the ancient ITS system is online and has
>>> even been resurrected so you can look at the code or even run it on 
>>> your
>>> modern computer - see https://github.com/PDP-10/its
>>>
>>> Jack Haverty (JFH at MIT-DM in the 70s)
>>>
>>> On 9/6/25 09:28, Guy Almes via Internet-history wrote:
>>>> Noel,
>>>>    So this was a real networked file system (and not just lots of 
>>>> FTP)?
>>>>    Very interesting,
>>>>      -- Guy
>>>>
>>>> On 9/6/25 11:35 AM, Noel Chiappa via Internet-history wrote:
>>>>>
>>>>>       > From: Guy Almes
>>>>>
>>>>>       > There are probably a number of ARPAnet sites where the 
>>>>> ARPAnet
>>>>>       > served this LAN role in the pre-Ethernet days.
>>>>>
>>>>> Notably MIT, where the 4 ITS machines shared their file systems 
>>>>> over the
>>>>> ARPANET.
>>>>>
>>>>>      Noel
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