Date: Mon, 3 Sep 2007 17:07:15 +1000 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Kris Kennaway <kris@FreeBSD.org> Cc: freebsd-questions@FreeBSD.org Subject: Re: g_vfs write error = 28, bad memory? Message-ID: <Pine.BSF.3.96.1070903152321.26352B-100000@gaia.nimnet.asn.au> In-Reply-To: <46DA9AC9.9090804@FreeBSD.org>
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On Sun, 2 Sep 2007, Kris Kennaway wrote:
> Ian Smith wrote:
> > On Sat, 01 Sep 2007 19:34:41 +0200 Kris Kennaway <kris@FreeBSD.org> wrote:
> > > Per olof Ljungmark wrote:
[..]
> > > > amavisd_enable="YES"
> > > > amavisd_ram="512m"
> > > >
> > > > and the line in rc.d/amavisd
> > > > mdmfs -M -s ${amavisd_ram} -w vscan:vscan md /var/amavis/tmp || true
> > > > for some reason creates a malloc based mfs
> > > >
> > > > Perhaps I should check this with the maintainer...
> > > >
> > > >
> > >
> > > Yes, malloc backing for md should be used in almost no situations.
> >
> > Am I right in thinking such situations would then be limited to diskless
> > / flashdisk / embedded systems having no swap? Seems obvious, but ..
>
> Sort of. Swap backing will still work when you have no swap, and it's
> still faster than malloc backing. The problem is that I think backing
> store reservation ("-o reserve") doesn't work unless you have actual
> swap to back everything, whereas with malloc backing it reserves in
> memory. This means that it is easy to overcommit memory and the system
> will probably panic when it suddenly finds no free memory for the md (as
> in the original email).
Ah. Swap backing with no swap configured sounded oxymoronic, and I was
confused and left guessing by md(4) on 5.5-STABLE (March) till checking:
http://www.freebsd.org/cgi/man.cgi?query=md&apropos=0&sektion=0&manpath=FreeBSD+7-current&format=html
which explains swap backed operation in one short but crucial sentence.
But is running out of memory with swap-backed md (with no swap) likely
to be any prettier than the panics from (unreserved) malloc backing?
> Ideally if no swap was configured, swap backing would also reserve the
> space in memory, and then I am not aware of any other reasons to
> continue using malloc backing.
By 'ideally' I guess you mean that it doesn't, yet? I hope to get a
Soekris 4801 before too long, which will provide a chance to experiment
(though I'll likely run it from one of my old 4GB laptop drives anyway).
Also noted in passing: the 'auto' parameter to bsdlabel(8) used by one
mdconfig(8) example is undocumented, though supported in bsdlabel.c
Thanks,
Ian
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