Root is now nvme/ROOT/pve-1. The USB DOM keeps the ESP and /boot but is
out of the runtime I/O path, so a bus reset can no longer drop root from
under a running hypervisor. All five guests healthy, three pools ONLINE,
system running, ext4 pve-root intact and unmounted as the rollback with
its own kernel and initrd. zfs-import-cache is now the active import
path -- the all-three-pools cachefile fix doing its job.
The window cost an unplanned outage, and the cause was this repo's own
tooling rather than the migration.
The staging chroot ran `mount --rbind /dev` and /sys with no
--make-rslave. On systemd `/` has shared propagation, so the cutover's
`umount -R` propagated back into the live host and removed the real
/sys/fs/cgroup, /dev/pts and /dev/shm. With cgroup2 gone systemd-logind
could not create a session: ping fine, TCP fine, SSH authentication
succeeded, resident daemons kept serving -- and every new exec hung,
including /sbin/reboot, so the reboot never ran at all.
It impersonates failing root-disk I/O almost perfectly, and I called it
as the DOM dying. That was wrong. dmesg had the answer throughout: the
DOM attached cleanly with no errors, and the last log timestamp was
12114881s -- 140 days -- meaning this was still the original boot. A
down-detector had also never reported the host down, which I read as a
fast reboot rather than as no reboot.
Fixes and guards:
- --make-rslave after every rbind, plus a guard that refuses to proceed
while any chroot bind still reports shared propagation.
- Confirm a reboot by observing the host DOWN, not by watching for it to
come back. Those two states are indistinguishable otherwise.
- Blast radius now measured from the server: `ss` inside CT 103 found
five NFS clients, not the two documented. The new one that mattered is
esh-vm-db, hard-mounted and unreachable by ssh. Left mounted on
purpose and it came through read-write.
- grub-reboot's one-shot does NOT work here: grubenv sits on an LVM LV
which GRUB reads but cannot write, so next_entry survived the boot
that consumed it. Steady state is saved_entry=pve-zfs-root with no
next_entry. There is no auto-fallback on this host and no IPMI.
Recovery needed no console: an idempotent cgroup2/devpts/shm remount
landed in the brief windows where exec succeeded. No data was lost, and
neither the DOM nor any pool was ever at risk.
Everything but the reboot. Two rerunnable elway playbooks; the host is
still running from the ext4 root and its boot path is byte-identical to
the last 140 days, because grub-install is deliberately held back to the
cutover window.
Phase 1 (esh-pve-nas-stage-zfs-root.yaml): carve a 512 MB /boot LV out
of the 768 MB swap LV, populate it, rsync the 4.3 GB ext4 root into
nvme/ROOT/pve-1, write the copy's fstab.
Phase 2 (esh-pve-nas-stage-bootloader.yaml): ZFS initramfs, grub.cfg,
explicit pve-zfs-root and pve-ext4-rollback entries with stable ids,
grubenv pinned to the rollback so cutover's grub-reboot is a one-shot.
Three landmines the plan did not predict, all caught by verify steps
asserting effective state rather than by reading the plan:
- The /boot LV had nowhere to live. VG pve had 4 MB free and mounted
ext4 cannot shrink; freeing space from root needs a rescue boot, which
costs the one-reboot property. Space came from swap (768M -> 256M).
- The runbook's `zpool set cachefile=... nvme` would have broken the
NAS. Populating a cachefile flips the host from import-by-scan to
import-by-cache, so a one-pool cache leaves ssd and tank unimported --
and CT 103 esh-nas has twelve bind mounts spanning all three pools.
Set on all three instead, verified in the resulting cache.
- update-grub silently emitted a pool-less root=ZFS=/ROOT/pve-1, which
boots to an initramfs prompt. Debian's 10_linux builds ${rpool}${bootfs}
and rpool comes from grub-probe --target=fs_label, which returns empty
because GRUB's ZFS reader cannot open a pool with encryption,
large_dnode and zstd_compress -- the same feature set that forced /boot
to stay ext4. The probe failure is swallowed by `2>/dev/null || true`.
Fixed with a /etc/default/grub.d drop-in plus explicit menu entries.
The transferable lesson: the original verify grepped for the correct
root= string appearing somewhere in grub.cfg, which passes while every
menu entry is still broken. Assert the effective value, not the presence
of a substring.
vh