Gentoo and read-only root file system.

cwr · Veteran Joined: 17 Dec 2005 Posts: 1969

I need to run Gentoo from an SD Card, and since these are notoriously failure-prone I'd like
to make the card read-only and use a ramdisk for /var, which as far as I can tell is the only
place which has to be writeable.

Someone's done this before - are there any major snags I should watch for?

Thanks - Will

NeddySeagoon · Posted: Wed Jul 03, 2013 6:57 pm Post subject:

cwr,

For root to be read only you need to make /etc/mtab a symlink to /proc/mounts
Thats not needed if you will tolerate /etc/mtab being written.

You will need to put /tmp in RAM too
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

slis · Posted: Wed Jul 03, 2013 8:05 pm Post subject:

I have prepared this kind of system about year ago (with kernel 3.2.6). It was based on gentoo, and I've made it 346MB in total.
As far as I can remember, I've done this like this:
1. prepare VirtualBox machine and install some base gentoo on it
2. copy all of this into directory (devel) where I can chroot and update & install packages
3. do some size-optimizations, such as busybox symlinks, remove unnecessary libraries and so
4. copy all necessary files into new directory (release) - that skips linux source, portage tree and some more
5. create two squashfs filesystems - root and usr (separated)
6. prepare disc images (loop filesystems) and copy kernel & root.squashfs into first, and "burn" usr into second
7. this image can be dd'ed into real device (in my situation, it was CF-card)

I have also done some sort of auto-backup, so the system may recover, when something will broke. As I can remember now, I've done some tests, where I've manually damaged filesystem, and all goes auto-recovered.

fstab on this looks like this:

cwr · Veteran Joined: 17 Dec 2005 Posts: 1969

slis - very, very many thanks. That gives me a clear starting point and a good idea
of the way ahead, just what I was looking for.

Will

steveL · Posted: Sat Jul 06, 2013 6:05 am Post subject:

Yeah, nice one slis. Added to my favourite tips under Advanced usage.

I for one would be interested to see your script.

slis · Posted: Sat Jul 06, 2013 9:38 am Post subject:

Thanks!

I did not expect such an interest.

As I said if someone would want, then I may publish some more details. But they are very solution-specific, so it may need much improvements before it would be useful for you. Also, the system is quite old for now, but that's the smallest problem

steveL · Posted: Sat Jul 06, 2013 6:35 pm Post subject:

slis · Posted: Sat Jul 06, 2013 7:13 pm Post subject:

If someone will help - then, why not?

fun fact: this system was running one Python+QT app, which was auto-started (something like kiosk?) So, if someone will need only console for example, without all X11 & qt stuff, it'll have huge size benefits.

I'll try to refresh this topic and publish something for few weeks, just after I'll finish my current task. I'll let you know here.

trumee · Guru Joined: 02 Mar 2003 Posts: 551 Location: London,UK

This is exactly what i am trying to do. Did you get around to posting the details?

slis · Posted: Fri Oct 31, 2014 7:54 pm Post subject:

It's nice to know that it still usefull for someone.

I'm currently working on preparing LinuxCNC + Gentoo system, which among others would contain: prepared images (about 300-400MB) with full system, and a script for creating one from scratch.

For now, I can paste only some very-very-developement version of script for creating image from directory with gentoo inside.

If you're interested, it looks like this:

1. Download stage3 and portage
2. Create ``ROOT'' directory, make it like your regular '/' (mkdir usr mnt dev proc sys tmp lib etc var bin sbin)
3. Unpack stage3 and portage as usual
4. You should be able to chroot inside. I, for convenience, prepared a script:

N8Fear · Posted: Sat Nov 01, 2014 7:46 pm Post subject:

I think the problem with SD-cards is vastly overstated.
I've been running Gentoo for about 1 1/2 years on a ultra cheap micro sd (from a german discounter called aldi - was something about 6 euro for 16 GB) on my raspberry pi. It's one of the first models with just 256MB ram, so I don't use tmpfs for anything except /dev while I even run emerge locally.
I have had no issues so far.
On the other hand I had 3 normal HDDs dying within 3 months after being bought. Just backup your data (for (small) SDcards dd is relatively quick so you can even have a pre-readied backup card at hand so you just need to reinstall userdata

slis · Posted: Sat Nov 01, 2014 8:05 pm Post subject:

I agree that when properly used sd/cf cards are good (safe enough).
Using read-only filesystems has several advantages and in some scenarios is the only real choice.

slis · Posted: Fri Dec 12, 2014 1:58 pm Post subject:

If someone is interested, I've created a script for creating gentoo-based system, which also is able to create raw image and write it directly into for example CF/SD card.

There is no WIKI page yet, but I'm working on it.

Basically, it's possible to create "blank" gentoo system, chroot into - look around and build what is needed, then build and write image.
There is no wiki page with tutorials, but --help should be sufficient.

https://sourceforge.net/p/gentoocnc/code/ci/default/tree/gaston/