Re: ancient OS history, ARM is sort of channeling the IBM 360

On Tue, 25 Jun 2024 11:24:03 -0400
EricP <ThatWouldBeTelling@thevillage.com> wrote:

Michael S wrote:

On Tue, 25 Jun 2024 08:49:43 +0200
Terje Mathisen <terje.mathisen@tmsw.no> wrote:
 

John Levine wrote: 

According to Lawrence D'Oliveiro  <ldo@nz.invalid>:   

How much of theoretical disk bandwidth was the filesystem capable
of using? Because I know early Unix systems were pretty terrible
in that regard, until Berkeley’s “Fast File System” came
along.   

My recollection is that if you were using QSAM with multiple
buffers and full track records it wasn't hard to keep the disk
going at full speed. Later versions of OS do chained scheduling
if you have enough buffers, doing several disk operations with
one cnannel program.   

Even on (MS)DOS it was easy to saturate the hard drive from a
single program, you just needed large enough (i.e. at least a full
track each) buffers.
 

 
I am not sure that "saturate the hard drive" is a correct wording.
According to my understanding, [when within track] hard drives used
in early PCs were more capable than hard disk controllers (Xebec
1210 in XT, I don't know what was used before XT). In turn, disk
side interface of disk controller was likely more capable than its
system bus side. Now, those are just feelings, I can't find hard
data to back it up.

I did end up making special file/record layouts which were
optimized for this, using exactly 4kB for each header+bitmap
record.
>
Terje 

 
According to this the interface to the ST506/ST412 drives from early
1980's could handle 5 Mb/s and the avg track seek time was 170 ms
(later 85 ms).
 
https://en.wikipedia.org/wiki/ST-506#History
 

Thank you. I didn't realize that PC HDs of early 80s were *that*
slow.

The 8-bit PC bus was 8 MB/s so there should be no
technical reason for not keeping *multiple* such drives fully busy
(seeking or transferring) while concurrently executing code.

8 MB/s sounds fast. I think, even for XT 2 MB/s is more realistic. Less
than that for original PC.