Re: 5.x: how do I get a *swap*-backed /tmp via rc.conf?

From: Lowell Gilbert (freebsd-stable-local_at_be-well.ilk.org)
Date: 10/10/05

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    To: stable@freebsd.org
Date: 10 Oct 2005 09:47:41 -0400

    David Wolfskill <david@catwhisker.org> writes:

    > I made the somewhat unexpected discovery that in FreeeBSD 5-STABLE,
    > if I use the "tmp*" variables in /etc/rc.conf to have an MFS /tmp
    > created, it is apparentyly not swap-backed -- as I expected from
    > the part of the mdmfs man page that reads:
    >
    > By default, mdmfs creates a swap-based (MD_SWAP) disk with soft-updates
    > enabled and mounts it on mount-point.
    >
    > and a review of the way mdmfs is invoked by /etc/rc; rather, it appears
    > to be malloc-backed.
    >
    > Here's what's in /etc/{defaults/,}rc.conf about it on one such machine:
    >
    > g1-59(5.4-S)[42] grep tmp /etc/{default*/,}rc.conf
    > /etc/defaults/rc.conf:tmpmfs="AUTO" # Set to YES to always create an mfs /tmp, NO to never
    > /etc/defaults/rc.conf:tmpsize="20m" # Size of mfs /tmp if created
    > /etc/defaults/rc.conf:tmpmfs_flags="-S" # Extra mdmfs options for the mfs /tmp
    > /etc/defaults/rc.conf:isdn_traceflags="-f /var/tmp/isdntrace0" # Flags for isdntrace
    > /etc/defaults/rc.conf:clear_tmp_enable="NO" # Clear /tmp at startup.
    > /etc/rc.conf:tmpmfs="YES"
    > /etc/rc.conf:tmpsize="512m"
    > /etc/rc.conf:tmpmfs_flags="-i4096"
    > g1-59(5.4-S)[43]
    >
    > And:
    >
    > g1-59(5.4-S)[43] sudo mdconfig -l -u md0
    > md0 malloc 524288 KBytes
    > g1-59(5.4-S)[44]
    >
    > Oh, for some sense of what we're working with:
    >
    > g1-59(5.4-S)[44] uname -a
    > FreeBSD g1-59.catwhisker.org. 5.4-STABLE FreeBSD 5.4-STABLE #6: Sun Oct 9 06:24:17 PDT 2005 root@g1-59.catwhisker.org.:/usr/obj/usr/src/sys/LAPTOP i386
    > g1-59(5.4-S)[45]
    >
    > So, back to my original question: how do I get a *swap*-backed /tmp?
    >
    > [The way I was alerted to the possibliity that my /tmp might not
    > be swap-backed is that I was using a machine running 5-STABLE as a CVS
    > server, in order to update /usr/ports on another machine. That's why I
    > have the -i4096 argument in there, BTW: to double the number of inodes
    > for when the CVS server starts consuming them with wild abandon as it
    > builds an isomorphic hierarchy to /usr/ports in /tmp. I think it only
    > took me 3 panics before I poked around in the mailing list archives and
    > noted scottl's comment about using a swap-backed /tmp instead, which got
    > me wondering what backing store my /tmp was using. I did look at
    > /etc/rc.d/tmp, as well a /etc/rc.subr, but I'm still failing to see why
    > I'm getting a malloc-backed /tmp. FWIW, although the /tmp in question
    > is an MFS in 4.x, I do this (use the box as a CVS server for /usr/ports)
    > without problem on 4.x.]

    It's hard-coded into rc_subr.
    Changing it to a default parameter and overriding it in rc.conf would
    probably be easy to get committed.
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