SUMMARY: Swap space requirements at 4.1.4

From: Lack Mr G M (
Date: Mon Jul 31 1995 - 11:27:13 CDT

     I asked about the swap requirements at SunOS4.1.4. I have a system
with 128MB of RAM and 64MB of swap (as /dev/sd0b).

     My thanks to those who sent me info. SunOS4 *does* require backing
swap space in order to use physical memory. So, with the setup above, I
was wasting about 64MB of memory. Since the system is up. running and
in use I didn't want to rejig the partitions to enlarge swap, so I have
just added a fs file (mkfile 64m sw0) as additional swap. This
definitely had an effect as the memory usage displayed by top dropped by
a lot as soon as I added this swap file (I suspect that what happened is
that the free, usable memory increased and that top displays total-free
as used).

     As well as appearing to have more usable memory I now should be
able to avoid;

> Aaaack! Virtual memory under SunOS use the "write on read" model,
> which requires that you have AT LEAST as much swap as RAM. When you try
> to access the 65th Meg of real memory SunOS will panic.

     I did get one report that:

> I should warn you that if your system panics, it will dump to /dev/rsd0b,
> and overwrite the next 64MB on the drive past the end of the b partition,
> which might take out /usr or something! This is one reason never to
> have more RAM than you do swap in /dev/rsd0b.

but my supplier assured me that the size of /dec/sd0b would be honoured
by the core dump and that I would just have a truncated core. I can
live with that.

PS: I don't actually expect this system to start paging, so I don't
wish to add more swap than I have physical memory.

My thanks to:

Robert Bonomi <>
Neil Hoggarth <>
Peter Carucci <>
Mark Thomas <>
Casper H.S. Dik <>
Patrick Stirling <>
Vince Angelo [ESRI-WashDC] <>

----------- Gordon Lack -----------------  ------------
The contents of this message *may* reflect my personal opinion.  They are
*not* intended to reflect those of my employer, or anyone else.

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