Re: old top and new -current: missing arcstat sysctl

On Wed, Aug 29, 2018 at 07:44:31AM +0200, Alexander Leidinger wrote:
> 
> Quoting Mark Johnston <markj_at_freebsd.org> (from Tue, 28 Aug 2018  
> 10:48:42 -0400):
> 
> > 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?
> 
> >> That is not something that a compat-xx package can handle.
> >
> > s/package/kernel option/?
> 
> Sorry, the COMPAT_FREEBSDx kernel options was what I had in mind when  
> I wrote this.
> 
> >> 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.
> 
> If the new top is summing the 3 up anyway, it sounds like we could  
> provide the old one as backwards compatibility, even if it is  
> redundant. I rather have an redundant counter and an old top working  
> (in the generic case of what we promise to our users; in this specific  
> case for me I just need to get around to update the jails on the  
> corresponding systems), than bailing out without displaying anything.

I'm inclined to agree, especially since this (running older top(1)s) has
come up before when I removed some VM sysctls:
https://reviews.freebsd.org/D16943