swap sizing

Greg 'groggy' Lehey grog at FreeBSD.org
Tue May 31 17:05:38 PDT 2005

On Tuesday, 31 May 2005 at  7:37:36 -0700, David O'Brien wrote:
> On Mon, May 30, 2005 at 04:11:55PM -0700, Kris Kennaway wrote:
>> On Mon, May 30, 2005 at 09:05:25AM -0600, Ken Gunderson wrote:
>>> On larger systems sporting several gigs of ram, what it the recommended
>>> swap scheme?  I'm aware of the 2x ram rule of thumb and also that this
>>> rule is considered "old school" by many.  And of course that you need
>>> at least as much swap as ram if you want to get a full dump...
> ..
>> I'd recommend as much swap as RAM (allowing for future expansion if
>> you think you might one day add more RAM).

It's interesting that nobody except bde has paid much attention to the
obvious fact that the more RAM you have, the less swap you use.  So
adding more RAM is only relevant in the scenario below:

> Plus some space for dump headers, etc..
> On an 4-32GB machines I use RAM+2MB.  I'm sure the dump headers is
> smaller, but if it grew one day...

Heh.  I've always danced around this value too.  We discussed it at
BSDCan last month; we didn't come to a firm conclusion either, but it
seems to be either 4 kB (probably) or 16 kB, repeated at beginning and
end.  So currently you'd get by with 32 kB more than memory size.

I personally use twice the memory size + 1 MB.  Nothing to do with the
old rule of thumb; the chance that I'll double the memory in any given
box is real, and reallocating dump partitions is a pain.

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