Suggest changing dirhash defaults for FreeBSD 9.2.

[Robert_Burmeister]

After scouring the internet, 
it seems that no one else has done a great deal of testing of UFS2 dirhash
defaults lately.

As the dirhash feature has effectively been tested for regressions,
I would like to propose setting the default dirhash values to my original
recommendation:

vfs.ufs.dirhash_maxmem: 20971520

vfs.ufs.dirhash_reclaimage: 60

The last benchmarks from 2008 only tested dirhash_maxmem at 2 and 64 megs.

I found in my testing that setting maxmem much over 32 megs makes the UFS
dirhash dysfunctional,
as scavenging 10% of maxmem on each vm_lowmem event becomes too aggressive.

While autotuning defaults is highly desirable, autotuning dirhash_maxmem
opens the can of worms of also tuning the hard coded 10% scavenging.

It may be better to hard code the maxmem default.


It may be appropriate to set the reclaimage to 5 minutes rather than 5
seconds;
I consider 60 seconds to still be a very conservative value, but at least an
effective one.




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