ZFS: unlimited arc cache growth?

Bernd Walter ticso at cicely7.cicely.de
Fri Apr 17 19:05:58 UTC 2009

On Fri, Apr 17, 2009 at 06:28:29PM +0200, Marius Nünnerich wrote:
> On Fri, Apr 17, 2009 at 16:18, Bernd Walter <ticso at cicely7.cicely.de> wrote:
> > On Fri, Apr 17, 2009 at 02:50:24PM +0200, Alexander Leidinger wrote:
> >> Hi,
> >>
> >> to fs@, please CC me, as I'm not subscribed.
> >>
> >> I monitored (by hand) a while the sysctls kstat.zfs.misc.arcstats.size
> >> and kstat.zfs.misc.arcstats.hdr_size. Both grow way higher (at some
> >> point I've seen more than 500M) than what I have configured in
> >> vfs.zfs.arc_max (40M).
> >
> > My understanding about this is the following:
> > vfs.zfs.arc_min/max are not used as min max values.
> > They are used as high/low watermarks.
> > If arc is more than max the arc a thread is triggered to reduce the
> > arc cache until min, but in the meantime other threads can still grow
> > arc so there is a race between them.
> Hmm, if this is true the ARC size should go down to arc_min once it
> did grow past arc_max and no new data is coming along but I do not
> observe such a thing here. It simply stays near but below arc_max here
> all the time. I have only /home on ZFS with moderate load.

I had a few ideas why this could be, but scanning complete sys showed
no point at all where arc_min is used.
There are formular to set this value, but that's all I find.

B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.

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