Relative performance of swap-backed MFS vs. regular UFS?

[Robert Watson]

On Sat, 23 Oct 2004, Igor Pokrovsky wrote:

> On Sat, Oct 23, 2004 at 01:12:46PM -0500, Ryan Sommers wrote:
> > >
> > >You can also use md(4). In my case I use it for /tmp.
> > >
> > MFS is the same thing as md(4). mfs = Memory File System, md = Memory 
> > Disk. Difference is only in the name.
> 
> I thought mfs is allocated from virtual memory, while md - directly from
> RAM.  Am I wrong? 

When you use mdconfig(8) to configure the md device, you get to specify
where you want the storage to come from.  The possibilities include malloc
(kernel address space + locked memory), but also vnode (file) and swap
(pageable memory).  Except during early netboot where swap is not
available, I would recommend against using malloc-backed disks.  For small
disks it's OK, but as they get large they chew into kernel memory and
address space, and so make it much easier for the system to run out of
kernel address space.

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Principal Research Scientist, McAfee Research