Date: Mon, 03 Sep 2007 10:26:11 +0200 From: Kris Kennaway <kris@FreeBSD.org> To: Ian Smith <smithi@nimnet.asn.au> Cc: freebsd-questions@FreeBSD.org Subject: Re: g_vfs write error = 28, bad memory? Message-ID: <46DBC523.3040707@FreeBSD.org> In-Reply-To: <Pine.BSF.3.96.1070903152321.26352B-100000@gaia.nimnet.asn.au> References: <Pine.BSF.3.96.1070903152321.26352B-100000@gaia.nimnet.asn.au>
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Ian Smith wrote:
> 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?
Probably not. No worse though.
> > 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).
Correct.
> Also noted in passing: the 'auto' parameter to bsdlabel(8) used by one
> mdconfig(8) example is undocumented, though supported in bsdlabel.c
OK, you should submit a PR.
Kris
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