Fwd: How do damaged root trees happen and how to protect against power cut?

[Carsten Behling <carsten.behling@googlemail.com>]

---------- Forwarded message ---------
Von: Carsten Behling <carsten.behling@googlemail.com>
Date: Di., 24. März 2020 um 08:51 Uhr
Subject: Re: How do damaged root trees happen and how to protect
against power cut?
To: Chris Murphy <lists@colorremedies.com>


Carsten Behling <carsten.behling@googlemail.com>

Mo., 23. März, 16:58 (vor 15 Stunden)
an Chris
> Seed device?
>
> Create a Btrfs file system, use space_cache v2,
> compress-force=zstd:16, and write the root image. Resize the file
> system to minimum. Set the seed flag. That's the base image. Part of
> the provisioning will be to 'btrfs device add' a 2nd partition, and
> remount read-write. This means two Btrfs file systems exist, each with
> their own UUID. You can reference the read-only seed by its UUID; and
> you can reference the read-write volume by its own UUID. On-disk
> metadata for this read-write volume points to both the read-only seed
> devid1, and the writable 2nd device devid2.
>
> Make sure write cache on the physical media is disabled.

Are this the correct steps in detail:

1. Partition SD card with:
- (write Bootloader ...)
- first partition boot (FAT32 (0x0b), 50MB)
- second partition (Linux Native (0x83), minimum possible size to fit rootfs)
- third partition (Linux Native (0x83), rest
- (write boot files (kernel ...))

2. Create seed device on development host:

# mkfs.btrfs --rootdir ~/rootfs --shrink /dev/sda2 # sda is my SD card device
# btrfstune -S 1 /dev/sda2
# dd if=/dev/zero of=/dev/sda3 bs=1024
# mount /dev/sda2 /mnt
# btrfs device add /dev/sda3 /mnt
# hdparm -W 0 /dev/sda3 # disable write cache

3. Mount on embedded device

- Kernel command line option: "root=/dev/mmcblk0p2 ro rootwait"
- Later, 'systemd-remount-fs.service' remounts seed device 'rw' by
appliying mmount options from fstab:
...
# 'defaults' includes 'rw', 'ROOT' is /dev/mmcblk0p2 (seed device)
LABEL=ROOT       /                    btrfs
defaults,noatime,nodiratime,space_cache=v2,compress-force=zstd:16
 1  1
...

Is this correct?

Regards
Carsten