[Info-vax] Kittson question

[terry+googleblog at tmk.com]

On Thursday, January 1, 2015 5:32:30 PM UTC-5, JF Mezei wrote:
> Don't emulators emulate only a subset of devices available on those
> systems ? For instance, wouldn't all disks in SIMH appear as "RA" drives
> (DUAx:) instead of SCSI (DKAxxx:) or DSSI (DIAx:) ?
> 
> Does an application really care if the disk is DUA0: DKA100: or DIA0: ?

For the case where we're talking about "directly-attached spinning rust", probably not - you can work around most of those issues with logical names. I'm sure some customer has hard-coded things in a way that there will be a problem if they try to move to an emulated environment.

Where things get more complicated is when the existing environment presumes some sort of access to a SAN - for example, Fibre Channel on Alpha. Moving such a system into an emulator presumes that the emulator can pretend to be the host adapter (an Emulex widget, IIRC) to the emulated VMS, and can also communicate to the external SAN via a (probably non-Emulex) FC host adapter in the host system. In addition to supporting both of those, there needs to be a way for the emulator to pass raw commands (as received from VMS) to the physical adapter, possibly translating data structures along the way.

Even when an emulator wants to talk to "vanilla" peripherals on the host system, some level of translation may be required. On my FreeBSD system running the AlphaVM emulator, AlphaVM can translate SCSI IDs/LUNs on-the-fly, as well as converting between 6-byte and 10-byte CDBs. It can also override the device info returned to VMS and emulate some of the quirks of VMS peripherals. I have been able to have a LTO library attached via SAS pretend to be a SDLT600 drive on a local SCSI bus in the emulated system. I've even tested using SATA drives in a RAID array on a SAS controller as a system disk. Note that this is using the physical drives; not normally done except for data migration purposes. Normally the VMS "disk drives" appear as simple files in the host system's normal filesystem, not on dedicated drives.