[Simh] VAX/VMS under v4.0

[Mark Pizzolato]

Hi Dave,

On Tuesday, November 17, 2015 at 2:22 PM, Dave Osborne wrote:
> Under V3.9-0 I have a VMS cluster running under SUSE 13.2.  I have two
> Vaxen running VMS 7.3 and an Alpha emulator running VMS 8.3 under
> FreeAXP. The Vaxen boot off a shared image disk with no problem and the
> Alpha boots from it's own disk. They all access the Ethernet device eth0 with
> no problem and I can access each system from any external system. Great!

Booting from a shared image disk has several potential issues.
#1) under v3.9 all I/O accessing the file which is the disk image is using stdio (instead of system calls read and write).  This means that there is local (in process) buffering which is not coherent between the different processes which may be accessing the same disk image file concurrently and thus updates may be inconsistent.  If the disk has been used extensively (for write activities) and there has been no corruption, then you've been lucky.  Under V4.0, the shared disk image can be accessed in "RAW" mode which uses system calls read and write directly and provides coherent access to the shared file.  
#2) VMS MUST be configured to know that the same disk image is being referenced by multiple systems concurrently (via under the covers hardware sharing instead of one system accessing the disk across the cluster via the MSCP disk server which would only have one hardware path to the disk).  How have you convinced both of these VMS systems that this is the case?

- Mark