[Info-vax] Command to show process rms file opens? (Enable disk caching)

[Roy Omond]

On 30/07/2020 13:52, David Hittner wrote:
> What is the command to show all RMS files being opened while the process executes commands? I recall seeing this in an "openvms internals" presentation somewhere, but can't remember what facility does this or how to invoke it.
> 
> I have an MMS build that runs 2 hours 21 minutes, and would like to see if there's anything that I could do to speed up the file access, such as INSTALLing frequently accessed files and images, moving files to faster access media, etc. But in order to generate a solution to the the problem I need to know exactly which files are being accessed.

You've already received a number of useful suggestions ...

Re "moving files to faster access media" - read on.

However you don't describe your system configuration in any way, so
what I write here may or may not be pertinent to your environment.

I'd say that most VMS users and even most system managers are unaware
that VMS explicitly disables disk caching for directly-connected SCSI
disks (yeah, I know many will not be using such disks, but many will).

At a few sites I've supported over the years until I retired at the
beginning of this month, I've achieved *huge* I/O performance
improvements by *enabling* on-board disk caching (this only applies
to SCSI-connected disks).  Seriously *huge* improvements ...

I am well aware that VMS likes to be sure that I/O writes have reached
the actual platter before considering the I/O write to be complete.
But this comes at a cost. I would guess that most system managers and
power users would readily accept the tiny risk (for some subset of
disks) associated with the possibility that the disk might hiccup just
when a write has been acknowledged by the disk but before the data are
actually on the platter.

Here's how to enable the on-board disk cache (note: the disk must have
already been mounted, since VMS disables it explicitly on mounting):

Example (an 18 Gbyte Compaq disk):

$ mcr sys$etc:scsi_mode -devname $2$dkb400 -page 8 -offset 0e 14 -mount 
-devtype "bf018863b4"

The important "bit" is the "4" at byte offset 0e in page 8 of the SCSI
mode pages.  Setting the bit on enables the on-board disk cache.

I don't have access to such a system at this moment, so I can't provide
any benchmark test results - any volunteers ?  My observations, though,
have always been "wow, that's very impressive".

Note also that has often been this "caution" that has given rise to the
perception that VMS I/O is slow (other systems have been running with
disk caching enabled).