On 9/28/2016 13:42, trafdev wrote:Thanks Jov and Karl!Primarycache=all is the default; changing it ought to be contemplated only under VERY specific circumstances. In the case of a database if you shut off "all" then an 8kb data read with 128kb blocksize will result in reading 128kb (the block size), returning the requested piece out of the 128kb and then throwing away the rest of the data read since you prohibited it from going into the ARC. That's almost-certainly going to do bad things for throughput! Note that having an L2ARC, which is the place where you might find setting primarycache to have a benefit, is itself something you need to instrument under your specific workload to see if its worth it. If you want to know if it *might* be worth it you can use (on FreeBSD) zfs-stats -E; if you're seeing materially more than 15% cache misses then it *might* help, assuming what you put it on is *very* fast (e.g. SSD) In addition if you're on FreeBSD (and you say you are) be aware that the vm system and ZFS interact in some "interesting" ways under certain load profiles. UMA is involved to a material degree in the issue. I have done quite a bit of work on the internal ZFS code in this regard; 11.x is better-behaved than 10.x to a quite-material degree. I have a patch set out against both 10.x and 11.x that address some (but not all) of the issues. |
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