Using mdconfig for swap space

[Jerry McAllister]

On Wed, Sep 09, 2009 at 11:23:14AM -0400, Lowell Gilbert wrote:

> Jerry McAllister <jerrymc at msu.edu> writes:
> 
> > 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.
> 
> Not necessarily.  What he wants is the '-t vnode' option for mdconfig(8).

Hmmm.   Haven't dealt with that before.   
Still seems like either a regular file or a dedicated partition
would be best.

////jerry


> 
> -- 
> Lowell Gilbert, embedded/networking software engineer, Boston area
> 		http://be-well.ilk.org/~lowell/