old top and new -current: missing arcstat sysctl

[Mark Johnston]

On Tue, Aug 28, 2018 at 10:25:39AM -0400, Allan Jude wrote:
> On 2018-08-28 02:40, Alexander Leidinger wrote:
> > Hi,
> > 
> > top reports missing sysctl kstat.zfs.misc.arcstats.other_size for
> > 12.0-alpha3 with a top from an old-ish -current.
> > 
> > Is/will this be handled via a compat-11 sysctl (my kernel is without
> > compat-xx), or did this slip through?
> > 
> > Bye,
> > Alexander.
> > 
> 
> That is not something that a compat-xx package can handle.

s/package/kernel option/?

> That arcstat was broken up into 3 individual stats, which the
> 12.0-alpha3 version of top will sum together for you.
> 
> I don't think we've had compat shims like this for previous versions of
> top, I recall having similar issues when the 'laundry' counter was
> introduced.

IIRC that would have been the inverted case of running a newer top(1)
with an older kernel lacking the v_laundry_pages sysctl.  In general I'd
expect us to support running an older top(1) with newer kernels if we
don't have to bend over backwards to provide compatibility.