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

["Gmane Admin" <gley-yocto@m.gmane-mx.org>]

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
 >
 >