Arch on USB key: Any way to overlay ramfs over / ?

Goran · 2012-01-24 06:32:45

I managed to install arch on a USB stick, by following the excellent instructions on the wiki, and I couldn't be happier.

Pacman is amazing. The configuration files are simple. The repositories are up to date.

However, the available documentation on minimizing writes, in order to prolong the life of a flash drive, seems a little sparse.

The common suggestions, such as adding in the "noatime" option, and moving browser profiles into /dev/shm, all seem like "half-solutions", when compared to the possibility of writing changes to a ramfs that overlays /, which can then be periodically merged to /.

In the bit of googling that I did so far, I managed to find references to a "union filesystem", such as aufs, which seems to be designed for exactly that - spreading one system over another, with the ability to do a merge at some later point in time.

So, I'm wondering if anyone here did something like that, and if you could provide me with some guidence as to how I could do the same.

Thanks.

ewaller · 2012-01-24 06:50:36

Not a direct answer to your problem, but have you seen this article ??

Edit: Never mind, you did

Last edited by ewaller (2012-01-24 06:51:10)

Blµb · 2012-01-24 08:54:35

Yes, aufs3 will do this. But in order to be able to use it you'll have to apply its patches to the kernel sources. (Also, for some reason I could only compile it as a module, and not directly into the kernel, but this shouldn't be an issue)
http://aufs.sourceforge.net/

There are packages in AUR, though linux-aufs_friendly seems to be flagged out of date atm. But you can still use it, or use the patches from the aufs3 standalone git repo. It contains a readme telling you exactly what to do.

My usecase for it was using squashfs for /usr while having a read-write branch still available to maintain full functionality.
This compressed my >12G /usr into a <4G squash-fs filesystem and reduced the HD access times by a lot, and it is probably also a good idea for a USB stick: squashfs with either rw ramfs, or a writable-branch on the stick. I think ubuntu even does that on its CD/USB sticks.

Once you got the module up and running, using it is quite simply, you mount your branches somewhere, say /mnt/sys/root /mnt/sys/ramfs, and then you combine them using `mount -t aufs -o br=/mnt/sys/ramfs=rw:/mnt/sys/root=ro none /` (assuming you're in the initrd and are about to mount / - or I hope you can directly mount it as /, otherwise you could mount it in a folder and than do mount --bind or something, I have never used initrds myself so I don't know how exactly you'd do that...)

Anyway, on that sourceforge page there's everything on how to use under "4. usage"

Goran · 2012-01-25 07:12:54

Blµb wrote:

Yes, aufs3 will do this. But in order to be able to use it you'll have to apply its patches to the kernel sources.

I was hoping that I could simply install a ready-made package, and add some options to /etc/fstab ... Guess I can give this a try when I get a little more familiar with my new environment.

Once you got the module up and running, using it is quite simply, you mount your branches somewhere, say /mnt/sys/root /mnt/sys/ramfs, and then you combine them using `mount -t aufs -o br=/mnt/sys/ramfs=rw:/mnt/sys/root=ro none /` (assuming you're in the initrd and are about to mount / - or I hope you can directly mount it as /, otherwise you could mount it in a folder and than do mount --bind or something, I have never used initrds myself so I don't know how exactly you'd do that...)
Anyway, on that sourceforge page there's everything on how to use under "4. usage"

Yes, I read that, and I think it's very informative, but it's not exactly a detailed guide on doing what I want.

How do I actually set this up, and where? In /etc/fstab, rc.conf, or what? Also, for a merge, is there a user space command, or do I have to set something up to run at some stage during shutdown (the former would be preferrable).

PS:

If anyone has any experience with using a USB stick as their primary storage medium, please share your experiences - What was your write minimization strategy (if any), and how long did the stick last?

Leonid.I · 2012-01-25 19:27:22

I think you're overcomplicating things.

1. Arch does not need to write to / per se, only /var. So you can have / and /boot mounted ro all the time.
2. In /var the only active place is /var/log which you can mount as tmpfs and sync every now and then with rsync. Alternatively, you could play with syslog-ng to enable logging of only error.
3. /var/cache/pacman can also be mounted separately and ro unless you want to update everyday.
4. /var/lib may contain a lot of small files, so choose an appropriate FS/mount options.
5. The rest of /var is still needed to be rw, but these are not active/constant writes. Moreover there is important data restoring which will be troublesome should your system crash. That's why I would keep it on a physical storage.

Arch Linux

#1 2012-01-24 06:32:45

Arch on USB key: Any way to overlay ramfs over / ?

#2 2012-01-24 06:50:36

Re: Arch on USB key: Any way to overlay ramfs over / ?

#3 2012-01-24 08:54:35

Re: Arch on USB key: Any way to overlay ramfs over / ?

#4 2012-01-25 07:12:54

Re: Arch on USB key: Any way to overlay ramfs over / ?

#5 2012-01-25 19:27:22

Re: Arch on USB key: Any way to overlay ramfs over / ?

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