[Simh] RE : RE : How to setup a simh VAX VMS cluster that handles more than 16 disks.

[Mark Pizzolato]

--- Boucher, François <boucher.francois at uqam.ca> wrote:


> >Of course, if you have a VAXcluster of SIMH instances running on a
single
> >host computer (e.g. a single PC running Windows or Linux), it 
might be
> >possible to have each simulated VAX connect to the same disk
image.  In
> >which case you would have shared storage and need to set ALLOCLASS
to some
> >positive integer > 0 (the same value on each machine which shares
the disk).
> 
> Effectively, I have a single desktop running 3 simh instances,
> because it contains a quad core CPU that permits this.  I did try
> to attach locally a disk image to more than one simulator instance,
> and unfortunately, it does not work!  I can mount the disk image in
> VMS on say two nodes, but if one node write any modification to it
> (let's say it creates a new file), the other simh instance is not
> aware of the data modification on the disk, as if the host OS
> (linux in my case) does not send a signal to the other node to make
> it aware that a change in the disk has occured.
> 
> The only way i found to successfully overcome this problem is to
> mount a given disk locally on only one simulated node, and to share
> it through MSCP to the other nodes in the cluster.  MSCP makes
> other nodes aware of modifications to the contents of a given disk.
>  Of course, you need a good network performance, and then you need
> to apply a tuning to the ethernet driving of simh.  (see the tread
> about [Simh] 10ms delay on ethernet communications - the solution!
>
<http://mailman.trailing-edge.com/pipermail/simh/2008-January/001539.html>
> )  However, my desktop is using a SATA disk, and it might be
> possible that using a fibre channel external disk permits this
> locally attached disk sharing.  This is a thing that SRI does with
> their Charon Vax setup.  I just did not test this for simh, so I
> cannot say true or false about this.  If anyone did successfully
> mount locally a disk image to two or more simh vaxes, please let me
> know how you did it!!!  I would be most happy to know about this.

This "could" work as is if the I/O done by simh to the virtual disk
container files was NOT done with the C runtime library (fopen,
fread, fwrite, fclose), and was done with direct unbuffered I/O to
the respective file storage.  Currently the buffering done by each
simh instance leads to inconsistencies since each simh instance has
its own buffered data which the other simh instances have no
mechanism to synchronize.

- Mark Pizzolato