/tmp on mfs

[Giorgos Keramidas]

On Tue, 20 Oct 2009 11:20:38 -0400, Christopher Hilton <chris at vindaloo.com> wrote:
> Reading the list yesterday I came across a little controversy about swap
> backed /tmp filesystems. I've been using this in my /etc/rc.conf
>
> tmpmfs="YES"            # Set to YES to always create an mfs /tmp, NO to never
> tmpsize="1g"            # Size of mfs /tmp if created
> tmpmfs_flags="-S -o async,noexec"       # Extra mdmfs options for the mfs /tmp
>
> To mount /tmp on a swap backed filesystem. I've been assuming that
> data stored in the /tmp directory was held in RAM and then written to
> the swap space only when the system had a more pressing need for the
> RAM. I typically configure my systems with swap == 2 * RAM or more.
> And on the systems in question I have at least 1Gb of RAM. I was
> hoping to use this trick to enhance the performance of the postgresql
> database (temp_tablespace=/tmp/pgsql/....) Is my assumption about
> where the data in a temporary file is stored incorrect?

Hi Christopher,

The rc.conf(5) options for `tmpmfs' and `varmfs' use the mdmfs(8) to
create the respective filesystems.  The default behavior of mdmfs(8) is
to create a swap-backed filesystem.  Swap-backed filesystems, as the
manpage of mdconfig(8) described them, work exactly as you wrote:

    swap     Storage for this type of memory disk is allocated
             from buffer memory.  Pages get pushed out to the
             swap when the system is under memory pressure,
             otherwise they stay in the operating memory.  Using
             swap backing is generally preferable over malloc
             backing.

So your understanding of the way tmpmfs behaves is fine.