[TUHS] LSX issues and musing

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Aug 1 05:57:02 AEST 2022

{I was going to reply to an earlier message, but my CFS left me with
insufficient energy; I'll try and catch up on the points I was goibf to make

    > From: Gavin Tersteeg

    > This leaves me with about 1.9kb of space left in the kernel for
    > additional drivers

I'm curious how much memory you have in your target system; it must not be a
lot, if you're targeting LSX.

I ask because LSX has been somewhat 'lobotimized' (I don't mean that in a
negative way; it's just recognition that LSX has had a lot of corners
trimmed, to squeeze it down as much as possible), and some of those trims
behind some of the issues you're having (below).

At the time the LSI-11 came out, semiconductor DRAM was just getting started,
so an LSI-11 with 8KB onboard and a 32KB DRAM card (or four 8KB MMV11 core
memory cards :-), to produce the 40KB target for LSX systems, was then a
reasonable configuration. These days, one has to really search to find
anything smaller than 64KB...

It might be easier to just run MINI-UNIX (which is much closer to V6, and
thus a known quantity), than add a lot of things back in to LSX to produce
what will effectively be MINI-UNIX; even if you have to buy a bit more QBUS
memory for the machine.

    > the LSX "mkfs" was hardcoded to create filesystems with 6 blocks of
    > inodes. This maxed the number of files on a disk to 96, but even with
    > the maximum circumvented LSX would only tolerate a maximum of 101 files.

And here you're seeing the 'lobotomizing' of LSX come into play. That '101'
made me suspicious, as the base V6 'caches' 100 free inodes in the
super-block; once those are used, it scans the ilist on disk to refill it.

The code in alloc$ialloc in LSX is hard to understand (there are a lot of
#ifdef's), and it's very different from the V6 code, but I'm pretty sure it
doesn't refill the 'cache' after it uses the cached 100 free inodes. So, you
can have as many free inodes on a disk as you want, but LSX will never use
more than the first 100.

(Note that the comment in the LSX source "up to 100 spare I nodes in the
super block. When this runs out, a linear search through the I list is
instituted to pick up 100 more." is inaccurate; it probably wasn't updated
after the code was changed. ISTR tis is true of a lot of the comments.)


   > A fresh filesystem that was mkfs'd on stock V6 can be mounted on LSX,
   > but any attempt to create files on it will fail. 

The V6 'mkfs' does not fill the free inode cache in the super-block. So, it's
empty when you start out. The LSX ialloc() says:

	if(fp->s_ninode > 0) {
	u.u_error = ENOSPC;

which would produce what you're seeing.

Also, another problem with trying to 'push' LSX into a previously un-handled
operating regions (e.g. large disks, but there are likely others) is that
there are probably things that are un-tested in that previously unused
operating mode, and there may be un-found bugs that you trip across.


    > Interestingly enough, existing large V6 RK05 images can be mounted,
    > read from, and written to. The only limitations on these pre existing
    > images is that if enough files are deleted, the system will randomly crash.

I had a look at the source (in sys4.c, nami.c, iget.c, rdwri.c, and alloc.c),
but I couldn't quickly find the cause; it isn't obvious. (When unlinking a
file, the blocks in the file have to be freed - that's inode 'ip' - and the
directory - inode 'pp' - has to be updated; so it's pretty complicated.)


   > The information there about continuous files ... will be extremely
   > helpful if I ever try to make those work in the future.

My recollection is that the LSX kernel doesn't have code to create contiguous
files; the LSX page at the CHWiki says "the paper describing LSX indicates
there were two separate programs, one to allocate space for such files, and
one to move a file into such an area, but they do not seem to be extant". If
you find them, could you let me know? Thanks.


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