[Simh] RT-11 Storage Strategy

[Johnny Billquist]

On 2016-02-21 16:51, Paul Koning wrote:
>
>> On Feb 21, 2016, at 7:32 AM, Johnny Billquist <bqt at softjar.se> wrote:
>>
>> On 2016-02-17 21:02, Paul Koning wrote:
>>>
>>>> On Feb 17, 2016, at 1:55 PM, Larry Baker <baker at usgs.gov> wrote:
>>>>
>>>> If you really want the RT-11 experience, try running it off RX01 floppies.  My first experience with DEC PDP-11's was a PDP-11/34 (not 34A; no FPU) running RT-11 off floppies.  Ka-klunk, ka-klunk!
>>>
>>> Yes, but that's still quite a lot faster than running it on DECtape.
>>>
>>> Still, the amazing part is that RT on DECtape actually works.  It's the only DEC OS I know of for which that is true.
>>
>> Like others said, lots of PDP-8 systems definitely ran only from DECtape. So that is not unique by any means.
>>
>> But more generally - DECtape is just a block addressable mass storage system, just like any other. In theory, any OS that can run from a disk can just as well run from DECtape. Speed will be slow, but that's the only thing about it.
>
> True in principle.  But a number of OS used a different file structure for DECtape than for disks -- because of the performance issues.  DOS does, for example, and RSTS copied the DOS DECtape directory structure (but not the disk one).  RT11, on the other hand, uses the same format for both, and the RT11 format works well.  That's why RT runs from DECtape without any extra work, while DOS and RSTS don't.

Interesting. I didn't know that RSTS/E used a different file system for 
DECtapes. RSX do not. It's the same file system used on both large 
disks, floppies and DECtapes.
RSX do have a few different options on how to deal with disks, but that 
has to do with if you have subdirectories on the device or not, and you 
can choose how the file system is done for any type of device. It's just 
a question of optimizing the layout for different purposes.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
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