Md based tmpfs with zfs root system.

Matthew Seaman matthew at
Thu Oct 19 05:59:07 UTC 2017

On 18/10/2017 17:45, Christopher Sean Hilton wrote:
> Good day,
> I've using a new system with root on zfs. I've traditionally used a
> /dev/md* based tmpfs for /tmp. A little initial reading on the subject
> shows an argument about this practice but I didn't find any threads
> that provide a resolution. Furthermore, all the information that I
> found which says that this is a bad idea dates from about FreeBSD-8.2,
> and that's long obsolete.
> What's the current resolution on this for a FreeBSD 11-STABLE user?
> Should I avoid using a swap backed /dev/md based tmpfs filesystem for
> /tmp?

Using a tmpfs for /tmp is perfectly fine -- but you don't need to touch
any /dev/md devices; just add a line like this to /etc/fstab:

tmpfs		  /tmp	      tmpfs    rw,mode=1777	0	0

I do this as standard on my machines, including in all jails.  In fact,
I think using a temporary swap backed filesystem for ephemeral data is
an underused idea in FreeBSD: machines nowadays generally have much more
swap configured than they will actually use, and that diskspace is
otherwise wasted.



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