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I have the `/tmp` partition of my OpenBSD system on a memory disk. Why? All of `/tmp` is cleared on a reboot anyway so I don't store valuable data there. Besides that, it speeds up access (more on that later) and I have 12G of RAM to waste. So why not waste it on a memory filesystem.
Although OpenBSD has support for *tmpfs* it is disabled by default since it's [not properly maintained](https://undeadly.org/cgi?action=article&sid=20160812011743) anymore. The other available memory filesystem is [mfs](https://man.openbsd.org/mount_mfs) - the memory filesystem (MFS) - that creates a RAM disk backed by your local swap partition.
I assume you have a swap partition, have you? If not, consider reinstalling and make sure that you have one. Find out your partition by using `swapctl`:
$ swapctl Device 1M-blocks Used Avail Capacity Priority /dev/sd1b 2048 0 2048 0% 0
Creating a MFS is as easy as mounting a regular FFS partition. Just specify any flags you want and provide the name of your swap partition. Here is an example mount where I mount a 2G MFS:
# mount_mfs -o nodev,nosuid,async -s 2048m /dev/sd1b /mnt/ # mount | grep mnt mfs:80942 on /mnt type mfs (asynchronous, local, nodev, nosuid, size=2048 1M-blocks)
Simply add an entry similar to the following to your `/etc/fstab` and make sure that the existing entry for `/tmp` is commented out. The maximum size is 2G and I use the usual mount flags. I also use *async* since the content will be lost upon reboot anyway.
# cat /etc/fstab | grep tmp swap /tmp mfs rw,nodev,nosuid,async,-s=2048m 0 0
Since MFS inherits the permissions from the root directory make sure that the permissions of the `/tmp` are 1777 (sticky bit). You can safely achieve this by rebooting into single user mode (reboot and type `bsd -s` at the loader prompt), fix the permissions and switch to multi-user mode.
One of my use cases to having `/tmp` on a MFS is to prevent that the Firefox disc cache ends up in `$HOME/.cache`. To set the non-standard location, start Firefox and navigate to `about:config`. Create a new string-based entry called `browser.cache.disk.parent_directory` and set the value to `/tmp`. Restart Firefox and you should see a new directory appearing. You can also check by navigating to `about:cache`.
Caution if you use multiple instances of Firefox running under different users. If you set the config value simply to `/tmp` the first running instance will create the cache directory and tighten the permissions so others cannot use it anymore. In this case, just set different path for each instance.
`$Id: mfs.gmi,v 1.1 2020/12/24 12:41:43 cvs Exp $`
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