[Simh] RT-11 Storage Strategy

[Johnny Billquist]

Rather late to the party, but a few comments might be useful anyway...

On 2016-02-17 14:43, Clem Cole wrote:
>
> On Tue, Feb 16, 2016 at 11:33 PM, Will Senn <will.senn at gmail.com
> <mailto:will.senn at gmail.com>> wrote:
>
> I do not remember the specifics, but I do remember that RL controllers
> had issues with Unix.   I suspect if you look in the driver you may find
> comments.

A bit surprised by that comments. The RL isn't particularly hard to work 
with, and usually do not cause problems. The biggest issue is that it's 
fairly primitive, in that it can only do relative seeks, and it can 
certainly fail getting to the right track when seeking. So the driver 
have to check and verity which track its on after every seek, and be 
prepared to do multiple seeks before any transfers.

But RL drives continue to this day to provide pretty reliable and 
working storage.

>     * Is there a controller that supports more disk devices than another
>     (RL vs RK, etc)?
>
> ​RK04/5/6 and RP04/5/6 controllers certainly support 8 units.
> My memory is RL can only support 4 (check the bits in the Unix driver -
> it should be pretty easy to see).   Dan Klein and I wrote the original
> RK611 driver for Unix by hacking on the RP06 driver.   My memory is that
> it supported 8 units also, but we only had 2 on our system.

RK04 I have never played with, so I can't comment on those.
RK05 originally allowed a max of 4 units, but there are two different 
RK11 controllers. The newer model supported 8 units.
RK06/RK07 is (as you mentioned) a different system altogether, rather 
similar to the RL system. But yes, the RK06/RK07 supports a maximum of 8 
units per controller.
RP drives, as well as RM, are massbus disks, which all support 8 units 
per controller.
(Actually, with RP, I should say RP04 and up. RP01,RP02,RP03 were not 
massbus disks, and is a totally different story.)

>     * Does one device have more capacity than another (either via single
>     disk raw capacity or via overall capacity of attached units)?
>
> ​Yes, RP06's were the largest until the SCSI drives show up.  But I did
> not think RT-11 supported them because it's file system would have
> overflowed.  Unix did it by partitioning the physical disk into logical
> disks.   Since Unix could had a uniform namespace, by mounting the
> logical disk it could piut the back together​ so the only downside was
> whatever limit the OS had for the largest file.  RT-11 could not do
> that, so if you partitioned the drive it, you would have DSK C:, DSK D:,
> DSK E: ...

No. RP06 was not the largest capacity. The RP07 was. And you also had 
the RM05. Both with larger capacity than the RP06.

I don't think RT-11 supported either the RM05 nor the RP07 though, but I 
think it might have supported the RP06. But yes, the file system size 
limitations of RT-11 makes it a bit silly. I know that for some disks 
RT-11 split the disk into several partitions, or logical disks, in order 
to use all of the disk. Not sure if it did this with the large massbus 
disks though.

>     * Is one device/controller more reliable in SimH than another?
>
> ​No idea.   ​

I would say that in the simh world, reliability is the same for everything.

>     * Do disks need to be formatted before initializing?
>
> Formatted no, initialized yes.​
>
> Formatting is a low level process that sets up the raw drive, regardless
> of OS.   Its make the physical disk pack recognizable to the physical
> controller (i.e. turning it in a set of blocks addressed with HD, CYL,
> SEC).   This was usually performed with a standalone program that was
> part of the disk controller diagnostics.
>
>
> Initialization is putting an OS specific file structure on the drive so
> the logical vector of disk sectors are interpreted as a file system.  In
> older Unix this is mkfs command, in newer versions, newfs.

Yes. The usual confusion of the "modern" PC users. Thanks to Microsoft, 
people have no idea about the difference between formatting and 
initializing.

>     * Are there some known best practice configurations (so many RL
>     controller, with so many drives, or so many RL and so many RK, etc.)?
>
>
> ​A typical small system install, had 2, maybe 4 RK05 that were
> dismountable.   DEC later created a version of the RK05 that was not
> dismountable, but could get twice the storage density.  A lot of
> 11/34A's were sold with the configuration.​

I would also say that when the RL disks came out, the RK05 became pretty 
outdated, and not seen much. The RL had larger capacity, was easier to 
connect, easier to configure, and the disk pack was also a bit better to 
handle.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol