Making an Ubuntu live USB stick persistent (Shallow Thoughts)

Akkana's Musings on Open Source Computing, Science, and Nature.

Fri, 28 Oct 2011

Making an Ubuntu live USB stick persistent

I wrote a few days ago about my multi-distro Linux live USB stick. Very handy!

But one thing that bugs me about live distros: they're set up with default settings and don't have a lot of the programs I want to use. Even getting a terminal takes quite a lot of clicks on most distros. If only they would save their settings!

It's possible to make a live USB stick "persistent", but not much is written about it. Most of what's written tells you to create the USB stick with usb-creator -- a GUI app that I've tried periodically for the past two years without ever once succeeding in creating a bootable USB stick.

Even if usb-creator did work, it wouldn't work with a multi-boot stick like this one, because it would want to overwrite the whole drive. So how does persistence really work? What is usb-creator doing, anyway?

How persistence works: Casper

The best howto I've found on Ubuntu persistence is LiveCD Persistence. But it's long and you have to wade through a lot of fdisk commands and similar arcana. So here's how to take your multi-distro stick and make at least one of the installs persistent.

Ubuntu persistence uses a package called casper which overlays the live filesystem with the contents of another filesystem. Figuring out where it looks for that filesystem is the key.

Casper looks for its persistent storage in two possible places: a partition with the label "casper-rw", and a file named "casper-rw" at the root of its mounted partitions.

So you could make a separate partition labeled "casper-rw", using your favorite partitioning tool, such as gparted or fdisk. But if you already have your multi-distro stick set up as one big partition, it's just as easy to create a file. You'll have to decide how big to make the file, based on the size of your USB stick.

I'm using a 4G stick, and I chose 512M for my persistent partition:

$ dd if=/dev/zero of=/path/to/casper-rw bs=1M count=512
Be patient: this step takes a while.

Next, create a filesystem inside that file. I'm not sure what the tradeoffs are among various filesystem types -- no filesystem is optimized for being run as a loopback file read from a vfat USB stick that was also the boot device. So I flipped a coin and used ext3:

$ mkfs.ext3 /path/to/casper-rw
/path/to/casper-rw is not a block special device.
Proceed anyway? (y,n) y

One more step: you need to add the persistent flag to your boot options. If you're following the multi-distro USB stick tutorial I linked to earlier, that means you should edit boot/grub/grub.cfg on the USB stick, find the boot stanza you're using for Ubuntu, and make the line starting with linux look something like this:

    linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile quiet splash noprompt persistent --

Now write the stick, unmount it, and try booting your live install.

Testing: did it work?

The LiveCD/Persistence page says persistent settings aren't necessarily saved for the default "ubuntu" user, so it's a good idea to make a new user. I did so.

Oops -- about that Ubuntu new user thing

But at least in Ubuntu Oneiric: there's a problem with that. If you create a user, even as class Administrator (and of course you do want to be an Administrator), it doesn't ask you for a password. If you now log out or reboot, your new user should be saved -- but you won't be able to do anything with the system, because anything that requires sudo will prompt you for your nonexistent password. Even attempting to set a password will prompt you for the nonexistent password.

Apparently you can "unlock" the user at the time you create it, and then maybe it'll let you set a password. I didn't know this beforehand, so here's how to set a password on a locked user from a terminal:

$ sudo passwd username

For some reason, sudo will let you do this without prompting for a password, even though you can't do anything administrative through the GUI.

Testing redux

Once you're logged in as your new user, try making some changes. Add and remove some items from the unity taskbar. Install a couple of packages. Change the background.

Now try rebooting. If your casper-rw file worked, it should remember your changes.

When you're not booted from your live USB stick, you can poke around in the filesystem it uses by mounting it in "loopback" mode. Plug the stick into a running Linux machine, mount it the usb stick, then mount it with

$ sudo mount -o loop /path/to/casper-rw /mnt

/path/to is wherever you mounted your usb stick -- e.g. /media/whatever. With the file mounted in loopback mode, you should be able to adjust settings or add new files without needing to boot the live install -- and they should show up the next time you use the live install.

My live Ubuntu Oneiric install is so much more fun to use now!

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[ 14:41 Oct 28, 2011    More linux/install | permalink to this entry | comments ]
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