Date: Fri, 17 Apr 2009 15:59:55 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: ticso@cicely.de Cc: Alexander Leidinger <Alexander@leidinger.net>, current@freebsd.org, fs@freebsd.org Subject: Re: ZFS: unlimited arc cache growth? Message-ID: <20090417205955.GK90152@dan.emsphone.com> In-Reply-To: <20090417190551.GT11551@cicely7.cicely.de> References: <20090417145024.205173ighmwi4j0o@webmail.leidinger.net> <20090417141817.GR11551@cicely7.cicely.de> <b649e5e0904170928p1568329bwb2f8a5f0f9b4f698@mail.gmail.com> <20090417190551.GT11551@cicely7.cicely.de>
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In the last episode (Apr 17), Bernd Walter said:
> On Fri, Apr 17, 2009 at 06:28:29PM +0200, Marius Nünnerich wrote:
> > On Fri, Apr 17, 2009 at 16:18, Bernd Walter <ticso@cicely7.cicely.de> wrote:
> > > On Fri, Apr 17, 2009 at 02:50:24PM +0200, Alexander Leidinger wrote:
> > >> 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.
zfs_arc_{min,max} are just tunables. The real variables arc_c_{min,max} get
autosized and then capped to {min,max} in uts/common/fs/zfs/arc.c:arc_init() .
--
Dan Nelson
dnelson@allantgroup.com
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