[Info-vax] V9.2-1 Page file size

[Jim Duff]

On 25/2/24 11:33, Chris Townley wrote:
> On 25/02/2024 00:12, Arne Vajhøj wrote:
>> On 2/24/2024 2:18 PM, Chris Townley wrote:
>>> Just upgraded a VM from 9.2-1 to 9.2-2 which seemed to have worked well.
>>>
>>> Bearing in mind this us just a hobbyist system, when I created the VM 
>>> I gave it 8GB memory, and an 8Gb system disk
>>>
>>> That was fine, and it created a 2Mb pagefile, not expecting it to be 
>>> used.
>>>
>>> The autogen run at the end of the upgrade obviously though this was 
>>> rubbish, so tried to create an 8Gb pagefile on the system disk. 
>>> Clearly it couldn't, so it created one of over 4Gb - leaving me with 
>>> 120Mb free.
>>>
>>> It also wanted an 8Gb dumpfile.
>>>
>>> Is any of this sensible?
>>
>> I believe that is one of the differences between Windows and VMS. On
>> Windows plenty of RAM and no/small pagefile works fine. VMS want
>> pagefile backing of virtual memory even if it is not likely to
>> need it. And running out of pagefile space is bad - very bad. So I got
>> 1 x 8 GB pagefile on my VMS x86-64 and 3 x 1 GB pagefile on
>> my VMS Alpha (can't remember what I have on my VMS Itanium).
>>
>> Arne
> 
> But VSI recommended 8Gb memory, and an 8Gb system disk. So how do we fit 
> that in? Especially if they want another 8Gb for the dump file
> 

In a production environment, by putting the page file (and secondary 
page files) and the dump file on disks other than the system disk.  The 
last thing you want is more paging to and from the system disk which 
does enough demand zero paging as it is due to image activation.  With 
real hardware, locally connected disks are nice for page files as long 
as you can provide redundancy for them (hardware mirroring with a raid 
controller, for example).

In a hobbyist situation, do you need a page file that large?  Unless you 
have an app that uses masses of memory, you can probably get away with 
something smaller.  I have 6GB of memory with 3GB of page file, and I 
rarely use more than 1% of the page file.  See warnings about no dump 
file and selective dumps in the documentation (qv) if you do this!

In a hobbyist situation, do you need a dump file?  Unless you're doing 
kernel mode development, perhaps not.  Set DUMPBUG=0 in MODPARAMS.DAT, 
reboot, and delete the dump file.

If you do want a dump file, you can probably get away with something 
much smaller by specifying an appropriate value for DUMPSTYLE in 
MODPARAMS.DAT.  You should at least compress the dump by setting bit 3, 
and choose other bits for that parameter based on your circumstances.

Remember, if a dump file doesn't exist either in SYS$SYSTEM: or in a 
DOSD location that you've specified, VMS will use the page file to write 
the crash dump.

There's an entire chapter devoted to all this in the "OpenVMS System 
Manager's Manual Vol 2".  Find it here:

https://vmssoftware.com/docs/VSI_SYS_MGMT_MANUAL_VOL_II.PDF

Jim.
-- 
eight-cubed.com