partition layouts and configuration

[Jimmy Tang]

Hi Alex

On Mon, Dec 15, 2014 at 11:12 PM, Alexander Sack <asac at canonical.com> wrote:
>
> On Mon, Dec 15, 2014 at 2:45 PM, Jimmy Tang <jcftang at gmail.com> wrote:
> > Hi All
> >
> > I'm trying to grasp the filesystem and partition layouts as described
> here
> > http://developer.ubuntu.com/snappy/filesystem-layout/
> >
> > How ever I wish to try and build an image for an arm based system
> (Novena),
> > the usual layout for the novena boards that I have (like all the imx6i
> based
> > systems) require that the first partition is reserved and formatted as a
> > fat32 partition for loading up the kernel.
> >
> > I couldn't find any information on how to shift the partition layout by
> one.
> > This is my desired layout
> >
> > - bootloader - /dev/sda1 - 512mb - fat32 partition
> > - system-a - /dev/sda2 - 2gb - ext 2/3/4 primary root filesystem
> > - system-b - /dev/sda3 - 2gb - ext 2/3/4 secondary root filesystem
> > - writable - /dev/sda4 - rest of the disk - ext 2/3/4 persistent user
> data
> >
> > How does one configure core/snappy to alternate between sda2 and sda3
> when
> > updates/reboots occur?
>
> Maybe its buggy, but in theory we are looking for partition label and
> not hard coded device paths. Setting those for your partitions
> accordingly might give what you want.
>
> In /usr/lib/python3/dist-packages/ubuntucoreupgrader/parser.py we have:
>
>   WRITABLE_DATA_LABEL = 'writable'
>
>   SYSTEM_DATA_A_LABEL = 'system-a'
>
>   SYSTEM_DATA_B_LABEL = 'system-b'
>
> ... and then ...
>
> def get_root_partitions_by_label():
>     '''
>     Returns a list of tuples of the recognised root filesystem partitions
>     available on this system. The tuples contain the following triplet:
>
>         ( <partition-name>, <full-device-path>, <mountpoint> )
>
>     '''
>     cmd = 'lsblk'
>     recognised = (SYSTEM_DATA_A_LABEL, SYSTEM_DATA_B_LABEL)
> ...
>
>
> I assume you have tried this?
>
>  -  Alexander
>

Ah okay that's clearer then, I'll be honest, I've been trying to dig up
more documentation details on how the system is implemented before
attempting to build my own image. I've not tried building an image just yet.

To follow up on this discussion how does snappy/core select the the
partition to boot up, is the mechanism tied to grub/syslinux? I ask this
because if the boot loader needs to be modified, I am restricted by
generating uboot scripts to change the rootfs when boot, although I can use
partition labels, I am still interested in how the boot sequence is
implemented.

Again is a publicly available design or architecture document on how it's
all implemented?

Jimmy


-- 
http://jcftang.github.io/ <http://www.sgenomics.org/~jtang/>
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