Date: Thu, 12 Aug 2004 11:28:14 -0400 From: Paul Mather <paul@gromit.dlib.vt.edu> To: freebsd-questions@freebsd.org Cc: Dan Nelson <dnelson@allantgroup.com> Subject: Re: tmpfs for FreeBSD? Message-ID: <1092324493.83481.39.camel@zappa.Chelsea-Ct.Org> In-Reply-To: <20040812120154.2EF1916A626@hub.freebsd.org> References: <20040812120154.2EF1916A626@hub.freebsd.org>
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On Thu, 12 Aug 2004 00:53:15 -0500, Dan Nelson <dnelson@allantgroup.com>
wrote:
> In the last episode (Aug 11), adp said:
> > I'm looking for a ramdisk-style filesystem for FreeBSD that can be
> > used for scratch space, e.g., tmpfs in Solaris. The filesystem should
> > be able to grow and shrink in memory (and use real disk space as
> > needed) depending on the amount of free RAM on the system. I don't
> > want just a fixed sized block of memory reserved for /tmp. I will be
> > using this for scratch files that are quickly created and then
> > destroyed, and will average around 2MB each. We are expecting out tmp
> > filesystem to need around 256MB to 512MB on average.
>
> The best available at the moment is a swap-backed filesystem. It will
> consume ram/swap as it grows, but won't release swap space when you
> delete files. If you're running 5.*, just put this in rc.conf:
>
> tmpmfs="YES"
> tmpsize="512m"
>
> and make sure you have at least 512MB of swap, so if it does happen to
> grow to full size and then have most of its files deleted, the free
> blocks can be pushed out to swap.
Alternatively, for 4.x (and 5.x) you can simply have an mfs /tmp entry
in /etc/fstab. Here is what I have in /etc/fstab for a 128 MB /tmp on
my 4.10-STABLE system:
swap /tmp mfs rw,nosuid,-s262144 0 0
On my 5.2-CURRENT system, I have this:
md /tmp mfs rw,-s128m 0 0
Note how the device to mount on is "swap" in 4.x and a "md" device in
5.x. Under 4.x, a "df -h" looks something like this:
Filesystem Size Used Avail Capacity Mounted on
[[...]]
mfs:28 124M 29K 114M 0% /tmp
and like this under 5.x:
Filesystem Size Used Avail Capacity Mounted on
[[...]]
/dev/md0 124M 12K 114M 0% /tmp
The "-s" option in the /etc/fstab mount entry determines the size of the
underlying VM device. In the case of the OP, that would be "-s512m" or
larger. Make sure there is at least as much swap space to back it.
FWIW, in 5.x you can also use a malloc-backed md device for a true RAM
disk.
Also, under 5.x, you can use a vnode-backed md device to use a large
regular file as underlying backing storage. This would be handy, I
presume, if you didn't want lots of your swap consumed by /tmp usage.
In 4.x, this would be accomplished via vn devices. Be careful of the
mount order when attempting this, though, if you want a virtual /tmp
created during boot.
See mount_mfs(8) for details and options.
Cheers,
Paul.
--
e-mail: paul@gromit.dlib.vt.edu
"Without music to decorate it, time is just a bunch of boring production
deadlines or dates by which bills must be paid."
--- Frank Vincent Zappa
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