[systemd-devel] Fragile journal interleaving

[Uoti Urpala]

The code that tries to determine which of two journal entries is
"earlier" (similar logic in journal_file_comparare_locations() and
compare_with_location()) does this by trying different progressively
less reliable ways to compare - first by seqnum if seqnum id matches,
then by monotonic time if boot id matches, then by realtime timestamp.
Unfortunately, while this method could be said to give the "right"
answer more often than just comparing by the always-available realtime
timestamp, it does not give a valid ordering relation. As in, unless
you can already rely on all those comparisons giving the same
consistent order (when you could probably just use realtime anyway),
it's possible that you get a relation like A

Re: [systemd-devel] "bootctl install" on mdadm raid 1 fails

[Bjørn Forsman]

On 9 December 2017 at 06:56, Andrei Borzenkov  wrote:
> [...]
> Firmware is unaware of MD RAID and each partition is individually and
> independently writable by firmware.

1. "Firmware is unaware of MD RAID". I agree.
2. "... independently writable by firmware". I don't expect firmware
to _write_ to the ESP (does it?!). As long as it only reads, nothing
will get out of sync.

> Pretending that you can mirror them
> on OS level is simply wrong.

I think that statement is correct for all md raid setups _except_
read-only access to raid 1 and with metadata 0.90 or 1.0 (superblock
at the end of device). Because in that case, a filesystem written on
the md array aligns with the underlying block device. So when the
system boots, EFI firmware can read /dev/sda1 and see the very same
filesystem that the OS put on /dev/md127.

Having ESP on an mdadm raid 1 array really works. (I now have a setup
of this myself.) But due to

  $ bootctl --path=/mnt/boot install
  Failed to probe partition scheme "/mnt/boot": Input/output error

, which my OS installer runs, it requires jumping through a few hoops
to get it running.

The hoops are:

1. Install OS with /dev/sda1 on /boot (no raid).
2. Setup /dev/md127 raid 1 on /dev/sdb1 with the 2nd device missing.
(May have to copy filesystem uuid from /dev/sda1 to /dev/md127.)
3. rsync filesystem contents from /dev/sda1 to /dev/md127.
5. Repurpose /dev/sda1 as the missing device in the /dev/md127 array
6. Use efibootmgr to create the 2nd boot entry, for /dev/sdb1.

I think these steps could be simplified/eliminated if "bootctl"
learned about mdadm (level 1) arrays.

Best regards,
Bjørn Forsman
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