Re: ZFS ARC tuning

On 9/13/2025 21:16, Garrett Wollman wrote:
> <<On Sat, 13 Sep 2025 04:02:01 +0100, Graham Perrin<grahamperrin@gmail.com> said:
>
>> Also, might one of the following be a better alternative?
>> vfs.zfs.arc_free_target
>> vfs.zfs.arc.sys_free
> The former *sounds* like it might be a reasonable lever, although I
> still feel like there's some sort of accounting issue here.  I note on
> my systems the latter is zero, but the former has a positive value
> (which seems to be related to memory size).
>
> -GAWollman

Have you looked at vmstat -z?

Specifically, the zfs-related ones.  Of particular interest is if the 
"Free" count for any of the sizes is unreasonably large.

There used to be a loooong time ago a problem with the zfs code and vm 
system interaction that under certain workloads the VM would not release 
free allocations back.  This would lead to severe pathological behavior 
including serious stalls, swap-outs and OOM kills.

I haven't had trouble with this in my workloads since the newer OpenZFS 
import occurred, but I'm curious if this is actually ARC not releasing 
memory when memory pressure occurs (that is, the space is actively 
allocated by ARC and is not being released back) OR if the space IS 
being released back but the VM system isn't freeing the space back.

If its the former then while ZFS should release it under pressure its 
possible the event happens fast enough that it doesn't respond in time 
and perhaps the knob to limit it will resolve it even though you 
shouldn't have to.  But if its the latter than twisting that knob is 
unlikely to bring joy.

-- 
Karl Denninger
karl@denninger.net
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