Observations from a ZFS reorganization on 12-STABLE

[Walter Cramer]

I suggest caution in raising vm.v_free_min, at least on 11.2-RELEASE 
systems with less RAM.  I tried "65536" (256MB) on a 4GB mini-server, with 
vfs.zfs.arc_max of 2.5GB.  Bad things happened when the cron daemon merely 
tried to run `periodic daily`.

A few more details - ARC was mostly full, and "bad things" was 1: 
`pagedaemon` seemed to be thrashing memory - using 100% of CPU, with 
little disk activity, and 2: many normal processes seemed unable to run. 
The latter is probably explained by `man 3 sysctl` (see entry for 
"VM_V_FREE_MIN").


On Mon, 18 Mar 2019, Pete French wrote:

> On 17/03/2019 21:57, Eugene Grosbein wrote:
>> I agree. Recently I've found kind-of-workaround for this problem:
>> increase vm.v_free_min so when "FREE" memory goes low,
>> page daemon wakes earlier and shrinks UMA (and ZFS ARC too) moving some 
>> memory
>> from WIRED to FREE quick enough so it can be re-used before bad things 
>> happen.
>> 
>> But avoid increasing vm.v_free_min too much (e.g. over 1/4 of total RAM)
>> because kernel may start behaving strange. For 16Gb system it should be 
>> enough
>> to raise vm.v_free_min upto 262144 (1GB) or 131072 (512M).
>> 
>> This is not permanent solution in any way but it really helps.
>
> Ah, thats very interesting, thankyou for that! I;ve been bitten by this issue 
> too in the past, and it is (as mentioned) much improved on 12, but the act it 
> could still cause issues worries me.
>
> -pete.
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