The effects of WITNESS and INVARIANTS

[Robert Watson]

On Tue, 24 Feb 2004, Willem Jan Withagen wrote:

> Just out of curriousity, and 'cause I've got some idle boxes, I started
> to do some (NFS-)performance testing.  There is still a long way to go,
> but I've but a first obvious result online running on the local server
> disk. 
> 
> It makes the claims of the effects of WITNESS and INVARIANTS very
> obvious.  Look especially at the graph for "Sequential block read". 
> 
> You might want to have a look at: 
>     http://withagen.dyndns.org/FreeBSD/nfs-performance/index.html.  Note
> that no NFS data is included.  I have some Bonnie-NFS data, but need to
> write accompanying test and conclusions for it. 
> 
> Suggestions are more than welcomed. 

My primary suggestion is "Turn off WITNESS and INVARIANTS when
benchmarking or for production systems".  We turn them off in releases,
and once 5.x becomes 5-stable, we'll turn it off by default also. However,
they're invaluable tools when debugging the development system, so we have
them on in the development branch by default.  I would encourage people to
generally run with them turned on unless performance of a system requires
them to be off, as it really helps the debugging process, as well as
helping to identify locking problems as the system evolves. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Senior Research Scientist, McAfee Research