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

[Francisco Ares]

Thanks, Gareth.

But it is very interesting that with no special customizations, I am almost
able to have a fully functional system.

That new thing about having to use a initramfs when the root filesystem and
the /usr directory are not present in the same physical partition, it turns
out that it helps on having the "genkernel" generated initramfs do the job
of mounting different arrangements for partitions, directories and unionfs
mounts. That is because of the file "/etc/initramfs.mounts" where the
specified mount entries, are extracted from the "/etc/fstab" and mounted
before the initram gives way to the real root environment.

For instance, in this system, for now, there is the directory structure
bellow:

/.var.rw  - here a r/w ext4 partition is to be mounted;
/.var.ro  - here there are all files and directories expected to be on a
usual /var;
/var - here a unionfs mount is to join the ones above

The part from fstab for this is ("/.var.ro" is present on the squashfs root
file system):

/dev/sda2               /                       squashfs
noatime,ro                              0 1
/dev/sda3               /.var.rw                ext4
noatime,defaults                        0 2
none                    /var                    unionfs
nonempty,cow,umask=022,dirs=/.var.rw/=rw:/.var.ro/=ro   0 0

I said it is not yet fully functional because it seems there is something
wrong with permissions, because it ends up with a unknown user prompt from
which I could list the root directories (something similar is being made to
"/etc"):

total 9,0K
drwxr-xr-x 23 root root  385 Out 18 14:18 ./
drwxr-xr-x  4 root root 4,0K Set 24 09:46 ../
drwxr-xr-x 47 root root 2,6K Out 17 16:03 .etc.ro/
drwxr-xr-x  2 root root   40 Out 18 15:09 .etc.rw/
drwxr-xr-x 11 root root  262 Out 17 16:03 .var.ro/
drwxr-xr-x  6 root root 4,0K Out 18 14:33 .var.rw/
drwxr-xr-x  2 root root 1,4K Out  7 14:46 bin/
drwxr-xr-x  4 root root 1,0K Out 18 14:15 boot/
d?????????  3 root root 2,3K Ago  1 00:17 dev/
drwxr-xr-x  1 root root 2,6K Out 17 16:03 etc/
drwxr-xr-x  3 root root   54 Ago 22 07:55 home/
lrwxrwxrwx  1 root root    5 Out  7 14:05 lib -> lib64/
drwxr-xr-x 12 root root 3,2K Out 17 08:51 lib64/
drwxr-xr-x  2 root root   28 Ago  1 00:16 media/
drwxr-xr-x  2 root root   28 Ago  1 00:16 opt/
drwxr-xr-x  2 root root    3 Jul 31 22:22 proc/
drwxr-xr-x  2 root root  136 Out 18 14:18 root/
drwxr-xr-x  5 root root  120 Ago 30 08:59 run/
drwxr-xr-x  2 root root 2,9K Out 17 08:51 sbin/
drwxr-xr-x  2 root root   28 Ago  1 00:16 sys/
drwxrwxrwt  2 root root    3 Out 18 14:18 tmp/
drwxr-xr-x 13 root root  267 Out 17 11:18 usr/
d?????????  1 root root  262 Out 17 16:03 var/

Any hints or suggestions? I will post this to the "gentoo-user" list, also,
I guess it is an interesting thing that might be called a cool side effect
of the recent issues.

Thanks
Francisco



2013/10/17 Gareth McClean <disneysw@×××××××.com>

> ** **
>
> 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****
>