On Fri, Sep 12, 2025 at 8:02 PM Graham Perrin <grahamper...@gmail.com> wrote:
>
> On 13/09/2025 03:25, Rick Macklem wrote:
>
> > …
> > - setting vfs.zfs.arc_max to a much smaller value than 99.9% and …
>
>
> vfs.zfs.arc_max is recognised, but legacy.
At least I can see this is used to set zfs_arc_max. See below..
> More modern:
>
> vfs.zfs.arc.max
I'm blind. I can't even figure out where/what this sets.
(If it also sets zfs_arc_max, I cannot find where in the code it does so?
Or does it set nothing at all?)
>
>
> Also, might one of the following be a better alternative?
>
> vfs.zfs.arc_free_target
Lets take this one as an example:
It appears to set the value of zfs_arc_free_target and then
this valriable gets used in a function called arc_available_target().
Now, here's the first lines of that function. (If you can guess what
this is supposed to be doing, you are better than I).
int64_t
arc_available_memory(void)
{
int64_t lowest = INT64_MAX;
int64_t n __unused;
/*
* Cooperate with pagedaemon when it's time for it to scan
* and reclaim some pages.
*/
n = PAGESIZE * ((int64_t)freemem - zfs_arc_free_target);
if (n < lowest) {
lowest = n;
}
A few observations...
- int64_t --> so they can be negative
- n __unused, but then it is
- compares n < lowest when lowest is set to the largest positive
value possible. (Wow, that seems useful??)
In general, it just sets lowest to "PAGESIZE * ((int64_t)freemem -
zfs_arc_free_target)"
without wondering what happens if freemem < zfs_arc_free_target, so n
is negative?
This is followed by some #ifdef'd code that I no idea if it is executed
or not.
So, at least I think I know what vfs.zfs.arc_max does and it seems to me
that it should be less than 99.9% of the machine's ram?
rick
ps: Like I said, there is not a lot in
sys/contrib/openzfs/module/os/freebsd/zfs/arc_os.c
and what there is looks kinda sketchy to me.
>
> vfs.zfs.arc.sys_free
>
>
