RE: [gentoo-embedded] planning a flash DOM x86 embedded system

[Gareth McClean]

I do this using a slightly modified init script, aufs3 and a squashfs.

 

Basically the system setups a tmpfs as a RW overlay on top of the squashfs. Then for permanent storage there is a physical Ext4 partition overlaid again on top of the /home directory. 

 

Symbolic links are then used for any configuration files (such as network configuration) that require persistent storage. These links obviously reference locations within the /home directory that is stored on an Ext4 physical partition.  

 

My init script is slightly more complicated than this since it caters for both live (squash plus aufs) and development (normal disc access) modes but basically it boils down to:

 

 

       mount -t aufs aufs "${NEW_ROOT}" -o dirs=/rw=rw:/ro=rr

 

       # Mount RW_PERM

       echo "Mounting RW"

       mkdir -p /rw-perm

       mount "UUID=${RW_PERM_UUID}" /rw-perm

       mount -o bind /ro/home "${NEW_ROOT}/home"

       mount -t aufs aufs "${NEW_ROOT}/home" -o dirs=/rw-perm=rw:/ro/home=rr

 

where 

ro = squashfs

rw =  tmpfs

rw-perm = physical Ext4 partition

 

Note you need the “mount -o bind" command to allow an overlay on top of an overlay with aufs3 otherwise you will get an error message.

 

BTW, Unless you have a good reason for doing this I would not recommend doing this since you have a physical hard drive. The reason I still do this is mainly for backward compatibility. Of course it is nice to have the ability to run our system from a liveUSB stick and it minimises software download size when doing updates but debugging the init script is a very time consuming process……

 

 

 

 

From: Francisco Ares [mailto:frares@×××××.com] 
Sent: 16 October 2013 13:49
To: gentoo-embedded@l.g.o
Subject: [gentoo-embedded] planning a flash DOM x86 embedded system

 

Hi.

I am planning to build a system to be deployed in a SATA flash disk, and most of the file system will be read-only. There will be a tempfs on /temp and a read-write partition for /var (perhaps a unionfs with the static part of /var and that read-write partition)

Is there any resources on how to do this using Gentoo?

There is already a development system with everything working as expected on the final system. But when I put it to a squashfs, the system boots with several errors, like when trying to write to /etc and /var.

Looking on the new issue regarding /usr and / being on a different partitions, I have found the file in /etc/initramfs.mounts. I have added the needed fstab entries to be mounted before the system switches to the real-root, (as the comments on top of this file claims)  but there are still errors during boot.

 

Thanks
Francisco