swap space issues

[Bob Bishop]

> On 26 Jun 2020, at 11:23, Peter Jeremy <peter at rulingia.com> wrote:
> 
> On 2020-Jun-25 11:30:31 -0700, Donald Wilde <dwilde1 at gmail.com> wrote:
>> Here's 'pstat -s' on the i3 (which registers as cpu HAMMER):
>> 
>> Device          1K-blocks     Used    Avail Capacity
>> /dev/ada0s1b     33554432        0 33554432     0%
>> /dev/ada0s1d     33554432        0 33554432     0%
>> Total            67108864        0 67108864     0%
> 
> I strongly suggest you don't have more than one swap device on spinning
> rust - the VM system will stripe I/O across the available devices and
> that will give particularly poor results when it has to seek between the
> partitions.

If you configure a ZFS mirror in bsdinstall you get a swap partition per drive by default.

> Also, you can't actually use 64GB swap with 4GB RAM.  If you look back
> through your boot messages, I expect you'll find messages like:
> warning: total configured swap (524288 pages) exceeds maximum recommended amount (498848 pages).
> warning: increase kern.maxswzone or reduce amount of swap.
> or maybe:
> WARNING: reducing swap size to maximum of xxxxMB per unit
> 
> The absolute limit on swap space is vm.swap_maxpages pages but the realistic
> limit is about half that.  By default the realistic limit is about 4×RAM (on
> 64-bit architectures), but this can be adjusted via kern.maxswzone (which
> defines the #bytes of RAM to allocate to swzone structures - the actual
> space allocated is vm.swzone).
> 
> As a further piece of arcana, vm.pageout_oom_seq is a count that controls
> the number of passes before the pageout daemon gives up and starts killing
> processes when it can't free up enough RAM.  "out of swap space" messages
> generally mean that this number is too low, rather than there being a
> shortage of swap - particularly if your swap device is rather slow.
> 
> --
> Peter Jeremy


--
Bob Bishop       t: +44 (0)118 940 1243
rb at gid.co.uk     m: +44 (0)783 626 4518





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