Using mdconfig for swap space

Daniel Bye danielby at
Wed Sep 9 15:48:12 UTC 2009

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:
> > 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:
> > > 
> > > > Are there any advantages to using mdconfig and creating a virtual disk for swap space as opposed to having a designated swap partition? For example, I could do something like this:
> > > 
> > > Unless I am missing something basic here, it seems like a bad idea to 
> > > me - to carve out and use up some memory to use as extra storage for 
> > > processes that need more memory that you have taken away to give to swap.  
> > > That is self defeating.
> > > 
> > > In addition, one use of swap is to write dumps to if there is a crash. 
> > > If you put it in memory, it is gone when you reboot.
> > 
> > He's talking about using a swap file, rather than a dedicated partition on 
> > the disk, not in RAM! Although it is slightly slower, as Chuck has already 
> > pointed out, it might, in certain circumstances, be a somewhat more 
> > convenient solution than repartitioning/reinstalling the whole system.
> > 
> > And as RW has said, the facility already exists and can be enabled with a
> > couple of knobs in /etc/rc.conf.
> 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 
> isn't the command he is trying to use (mdconfig) for creating a memory 
> filesystem - eg use a chunk of memory and make a file from it (then use it 
> 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.


Daniel Bye
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