Re: How to create a btrfs snapshot in the rootfs image (during build)?


Gmane Admin
 

Self answering below.

Op 22-02-2020 om 16:06 schreef Ferry Toth:
Hi all,

I am able to generate a rootfs image of type btrfs.
Btrfs supports subvolumes, for instance with name @.

Using subvolumes instead of partitions allow more efficient use of
the emmc compared to separate partitions. And it allows multiple versions of the rootfs in the same pool.

Manually I can create a subvolume (on the host) by loop mounting the
btrfs image and taking the snapshot. Something like this:
mkdir tmp
sudo mount edison-image.btrfs tmp
sudo btrfs subvolume snapshot tmp tmp/@

and then remove all dir's in root except @.

mount and btrfs operations need root permissions AFAIK. Is there any
way to add these to the recipe?

Up to now I found I can use udisksctl to mount the image as ordinary user.

However, to create a snapshot it appears I need to own the root directory and for that would need an option like mke2fs -E root_owner.

As that doesn't exist, the only workaround I find is to create an ext4 partition with -E root_owner and then convert to btrfs.

Is there really no better way?

There is 2nd thing I want to achieve:
Btrfs can send / receive these snapshots allowing over-the-air-updates.

For this I need to:
sudo btrfs subvolume snapshot -r tmp/@ tmp/@ota
sudo btrfs send tmp/@ota > edison-image.snapshot

And then later manually (with the device having mounted btrfs on /mnt):

cat edison-image.snapshot | ssh root@edison "btrfs receive /mnt"
ssh root@edison "btrfs property set -ts /mnt/@ota ro false"

How do I automated the first steps in yocto?

Ferry

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