On 06/01/2012 02:33 PM, Jeff Bacon wrote:
I'd be interested in the results of such tests. You can change the primarycache
parameter on the fly, so you could test it in less time than it
takes for me to type this email :-)
  -- Richard
Tried that. Performance headed south like a cat with its tail on fire. We 
didn't bother quantifying, it was just that hideous.

(You know, us northern-hemisphere people always use "south" as a "down" 
direction. Is it different for people in the southern hemisphere? :) )

There's just too many _other_ little things running around a normal system for 
which NOT having primarycache is just too painful to contemplate (even with 
L2ARC) that, while I can envisage situations where one might want to do that, 
they're very very few and far between.

Thanks for the valuable feedback Jeff, though I think you might misunderstand - the idea is to make a zfs filesystem just for the files being mmaped by mongo - the idea is to only disable ARC where there is double caching involved (i.e. for mmaped files) - leaving rest of the system with ARC and taking ARC out of the picture with MongoDB.

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