Date: Wed, 9 Sep 2009 16:46:56 +0100 From: "Daniel Bye" <danielby@slightlystrange.org> To: "'freebsd-questions@freebsd.org'" <freebsd-questions@freebsd.org> Subject: Re: Using mdconfig for swap space Message-ID: <20090909154655.GA96342@torus.slightlystrange.org> In-Reply-To: <20090909145922.GB22253@gizmo.acns.msu.edu> References: <7B9397B189EB6E46A5EE7B4C8A4BB7CB3037EBB7@MBX03.exg5.exghost.com> <20090908235259.GB19173@gizmo.acns.msu.edu> <20090909105707.GA27941@torus.slightlystrange.org> <20090909145922.GB22253@gizmo.acns.msu.edu>
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On Wed, Sep 09, 2009 at 10:59:23AM -0400, Jerry McAllister wrote:
> On Wed, Sep 09, 2009 at 11:57:07AM +0100, Daniel Bye wrote:
>=20
> > On Tue, Sep 08, 2009 at 07:52:59PM -0400, Jerry McAllister wrote:
> > > On Tue, Sep 08, 2009 at 04:51:20PM -0500, Peter Steele wrote:
> > >=20
> > > > Are there any advantages to using mdconfig and creating a virtual d=
isk for swap space as opposed to having a designated swap partition? For ex=
ample, I could do something like this:
> > >=20
> > > Unless I am missing something basic here, it seems like a bad idea to=
=20
> > > me - to carve out and use up some memory to use as extra storage for=
=20
> > > processes that need more memory that you have taken away to give to s=
wap. =20
> > > That is self defeating.
> > >=20
> > > In addition, one use of swap is to write dumps to if there is a crash=
.=20
> > > If you put it in memory, it is gone when you reboot.
> >=20
> > He's talking about using a swap file, rather than a dedicated partition=
on=20
> > the disk, not in RAM! Although it is slightly slower, as Chuck has alre=
ady=20
> > pointed out, it might, in certain circumstances, be a somewhat more=20
> > convenient solution than repartitioning/reinstalling the whole system.
> >=20
> > And as RW has said, the facility already exists and can be enabled with=
a
> > couple of knobs in /etc/rc.conf.
>=20
> I understand using a file and making it in to swapspace. I have used that
> a couple of times when I needed to add some swap space temporarily. But=
=20
> isn't the command he is trying to use (mdconfig) for creating a memory=20
> filesystem - eg use a chunk of memory and make a file from it (then use i=
t=20
> for swap or whatever)? That is in RAM.
No, with the -t vnode and -f <filename> options, he'd actually be creating
a file-backed memory disk. The terminology can be a little confusing, but
in this instance the file wouldn't be loaded into RAM, but would instead
be treated as any other disk-like device. It's exactly the same approach
as used by /etc/rc.d/addswap, which gets its configuration from $swapfile
set in /etc/rc.conf.
Dan
--=20
Daniel Bye
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