Simplest (practical) file system?

From: Jim Battle <frustum_at_pacbell.net>
Date: Mon Jul 28 00:31:01 2003

Bob Shannon wrote:

> This sounds just about right, vintage-wise!
>
> Ok, so once you have a hole, do you only recover that disk space by
> SQUEEZEing the disk?

yes, although the OS supplied command that does this can't do an
in-place squeeze. the command is MOVE, and it copies the first disk to
the second disk while removing any SCRATCH'd files along the way. I'm
sure people did write their own in-place squeeze programs, but it would
be inherently hazardous. Because the file system doesn't have a linked
list or indirection table of some kind, you can't incrementally move
around pieces of the file safely. You have to atomically move the
entire program and update the catalog. So the build in command, MOVE,
required the squeeze to happen during a whole-disk copy so that the
integrity of the source disk was never compromised.

> I think RT-11 works this way, at least I recall the SQUEEZE command
> making the disk free space
> larger.
>
> Is a FAT needed for this, or is there a simple way to calculate this on
> the fly?
>
> Jim Battle wrote:
>
>>>
>>
>> The wang 2200 that I have (pre 1975) has two modes:
>>
>> 1) direct access. the program and read or write any absolutely
>> address sector, which is 256 bytes. this isn't what you asked for.
>>
>> 2) random access tape. the first N sectors of the disk contain a
>> catalog of files on the disk. an entry consists of 8 bytes of
>> filename, two bytes of starting sector, two bytes of final sector (or
>> is it size?) and an attribute byte (program/data). it is quite
>> primitive. if you want to create a data file, you must first reserve
>> all the space you will ever need for it. unused parts of the file
>> still take up space. if the file does grow bigger than you had already
>> preallocated, you must basically shuffle files around on disk to make
>> room. when you delete a file, it just makes a hole in your map. all
>> sectors are allocated linearly.
>>
>> it is very simple, but not too convenient.
Received on Mon Jul 28 2003 - 00:31:01 BST