Using mdconfig for swap space

Lowell Gilbert freebsd-questions-local at
Wed Sep 9 15:23:16 UTC 2009

Jerry McAllister <jerrymc at> 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).

Lowell Gilbert, embedded/networking software engineer, Boston area

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