Changing the number of file headers on an RSX volume

Chris Zach cz at
Wed Jun 24 12:37:47 CDT 2020

I did get an answer from Johnny B, and it makes sense why I couldn't 
figure it out: In order to increase the file handles significantly you 
have to either dismount with "dmo du0:/dev/lock=v" to lock tasks in 
memory then mount foreign and back it up or just put BRU64K on an RX01 
floppy with VMRV48 and boot from that.

Then after the volume is backed up to tape you clobber it by using BRU 
to restore it and set the /MXF parameter on that line. Otherwise BRU 
keeps the same value for MXF and the files limit is still there.

The neat part (and why I couldn't remember) is I used to use DSC to back 
up my RL02's and RM02's to other RL/RM disk packs. DSC did a pretty much 
image copy while BRU actually creates a new file system and then copies 
the files and the boot information. I didn't use BRU at the time because 
I didn't have a tape drive.

Modern technology marches on. I'm waiting on a pair of TK50 tapes to 
arrive from TrashBay, once I have those checked out I'll do the backup 
and restore and get myself back in business.


* Dismounting the system volume on RSX11 is tricky. You can just 
dismount it, but then the system will lock you out of installed tasks 
and you'll be dead in the water. The /dev/lock=v holds the tasks in 
memory so you can still run your tools with the disk dismounted.

A quick way to do this is to bring up the system, then shutdown and when 
you get to ODT type P to proceed. The system will come up with the shell 
but no volumes.

Using HOM can increase the number of files or directories but only up to 
a point. Also with home you need to first set the device to private 
(only do-able when you do the shutdown and resume with P trick) then 
allocate the disk, then mount it foreign.

It's never dull in pdp11 world.


On 6/24/2020 12:57 PM, Kenneth Gober wrote:
> On Tue, Jun 23, 2020 at 9:03 PM Chris Zach via cctalk 
> <cctalk at <mailto:cctalk at>> wrote:
>     So I'm working on this RSX11M+ system here and while working I ran
>     myself out of file headers. Using the HOME /MXF command I was able to
>     increase the number of headers, but only up to 4090. or so. Trying
>     to go
>     to 4100 gave me an error saying there were not enough system blocks or
>     something. Currently I have 830 headers, but that's not enough in the
>     long term.
> I've never actually used RSX-11 myself, all I know about it is what I read
> in the ODS-1 spec, but I'm guessing the issue here is that the volume is
> set up with only 1 block allocated for the index file bitmap, and growing
> the number of file headers beyond 4096 would require growing the index
> file bitmap to 2 blocks (or more, depending on how many file headers you
> expect to need).  Since the first 16 file headers must follow directly after
> the index file bitmap, growing the bitmap requires moving those first 16
> file headers as well, which in turn requires moving whatever may currently
> be in the way.  None of this of course is something I'd expect the system
> to be able to do while the volume is mounted.
> What precisely you have to do to achieve the goal of having a volume
> with more file headers I can't say, sorry, but I thought I could at least
> provide some extra context for other readers on why it's not as trivial to
> do as one might guess.
> -ken

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