The mysterious kern.maxswzone

Andrea Venturoli ml at
Tue Jan 22 07:47:05 UTC 2019

On 1/21/19 9:44 PM, Kevin P. Neal wrote:

> With the difference in performance between a CPU and disk, does relying
> on swap space make sense anymore?

I think it still does.
Of course performance will be terrible, but it can be acceptable for 
short periods of time: e.g. I've got a box which will swap considerably 
when periodic is run, but that happens at night and no one will notice.

Also, I prefer an occasional slow down to processes being occasionally 

If the machine constantly and heavily swapping, it's time for either a 
software diet or hardware upgrade.

> The x1 and x2 rules of thumb come from a different era.

One thing to keep in mind are crash dumps: they are written in a swap 
partition, so, if you don't have enough and your box panics, you won't 
be able to debug it.
I've been hit in the past by this: a box's swap was be taylored to RAM 
size, the RAM was doubled and crash dump could not be obtained anymore.

I also heard something about "minidumps", or compressed dumps, but did 
not investigate... so yes, all this *might* be obsolete; however, with 
modern HD sizes (1TB at the least) I prefer to "waste" some more space 
for swap, than to be sorry later.

That's also why I don't bother about kern.maxswzone: I have some swap 
space I don't need *now*; it's just sitting there for eventual future 
needs :)

> Oh, and won't swapping to a GEOM mirror cause deadlocks? I thought so, but
> I wouldn't take my word for it.

I have several boxes with swap on GMIRRORs and I've never seen any 
deadlock due to this fact.
Just my experience... perhaps there are corner cases?


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