Fix SD mirroring of zram-managed data and first-boot zram failure

Address findings from community test bench report (#2156):

- Sync SD (diff mode) rsynced live /var/log and openHAB persistence into
  /opt/zram/*.bind on the target, but zram-config bind-mounts the target
  directory over those paths at boot, so the synced data was shadowed and
  never used. Worse, the main rsync's --one-file-system + --delete emptied
  the clone's lower /var/log and persistence directories because they are
  overlay mountpoints on the running system. Rsync now targets the plain
  lower paths that zram-config uses as overlay lowerdir.

- Both raw and diff copies carried over the live system's
  /usr/local/lib/zram-config/zram-device-list, causing zram-config on the
  clone's first boot to skip device creation. The state file is now
  removed from the target in both paths.

- Run sync and udevadm settle between dd and set-partuuid to avoid
  intermittent PARTUUID assignment failures on loaded systems.

- Document that raw copies cannot capture data still held in zram.

Fixes #2156

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ecdye

docs/openhabian-backup.md

@@ -41,6 +41,9 @@ If you can't find the same model, play it safe and buy a model larger than what
 We also provide a couple of different options to allow you to backup your system to a local NAS, cloud storage, or a second SD card.
 For more information on these options, please see [Storage Preparation](#storage-preparation).
 
+Note that the semiannual raw copy reads the SD card at the block level, so any log and persistence data still held in ZRAM (i.e. written since the last ZRAM sync or reboot) will not be part of the raw copy.
+The daily rsync run covers this by copying the live view of those directories, so a mirror card will at most be a day behind on ZRAM-held data.
+
 ##### Moving the Root Filesystem
 
 Moving your system root to a USB stick or SSD is unsupported and dangerous.

functions/backup.bash

@@ -416,6 +416,9 @@ mirror_SD() {
     if ! cond_redirect dd if="${src}p1" bs=1M of="${dest}1" status=progress; then echo "FAILED (raw device copy of ${dest}1)"; dirty="yes"; fi
     if ! cond_redirect dd if="${src}p2" bs=1M of="${dest}2" status=progress; then echo "FAILED (raw device copy of ${dest}2)"; dirty="yes"; fi
     sfdisk -d ${src} | grep -q "^${src}p3" && if ! cond_redirect dd if="${src}p3" bs=1M of="${dest}3" status=progress; then echo "FAILED (raw device copy of ${dest}3)"; dirty="yes"; fi
+    # flush kernel buffers and let udev settle so blkid/mount see the freshly written partitions
+    cond_redirect sync
+    cond_redirect udevadm settle
     origPartUUID="$(blkid "${src}p2" | sed -n 's|^.*PARTUUID="\(\S\+\)".*|\1|p' | sed -e 's/-02//g')"
     if ! partUUID="$(yes | set-partuuid "${dest}2" random | awk '/^PARTUUID/ { print substr($7,1,length($7) - 3) }')"; then echo "FAILED (set random PARTUUID)"; dirty="yes"; fi
     if ! cond_redirect e2fsck -f -y "${dest}2"; then echo "FAILED (e2fsck)"; dirty="yes"; fi
@@ -428,6 +431,8 @@ mirror_SD() {
     mount "${dest}2" "$syncMount"
     sed -i "s|${origPartUUID}|${partUUID}|g" "$syncMount"/etc/fstab
     [[ -f "${syncMount}/etc/systemd/system/${storageDir}".mount ]] && sed -i 's|^What=.*|What=/dev/mmcblk0p3|g' "${syncMount}/etc/systemd/system/${storageDir}".mount
+    # remove zram state captured from the live system, else zram-config skips device creation on the clone's first boot
+    rm -f "${syncMount}/usr/local/lib/zram-config/zram-device-list"
     umount "$syncMount"
     if ! cond_redirect fsck -y -t ext4 "${dest}2"; then echo "OK (dirty bit on fsck ${dest}2 is normal)"; dirty="yes"; fi
     if [[ "$dirty" == "no" ]]; then
@@ -453,8 +458,12 @@ mirror_SD() {
     mount "$dest" "$syncMount"
     if ! (mountpoint -q "$syncMount"); then echo "FAILED (${dest} is not mounted as ${syncMount})"; return 1; fi
     cond_redirect rsync --one-file-system --exclude={'/etc/fstab','/etc/systemd/system/*.mount','/opt/zram','/srv/*'} --delete -aKRh "/" "$syncMount"
-    cond_redirect rsync --one-file-system --delete -aKh "/var/lib/openhab/persistence/" "${syncMount}/opt/zram/persistence.bind"
-    cond_redirect rsync --one-file-system --delete -aKh "/var/log/" "${syncMount}/opt/zram/log.bind"
+    # sync the live overlay (merged) view into the clone's lower dirs, i.e. the plain paths zram-config
+    # uses as lowerdir on boot; the *.bind mountpoints are shadowed by bind mounts and must not be targets
+    cond_redirect rsync --one-file-system --delete -aKh "/var/lib/openhab/persistence/" "${syncMount}/var/lib/openhab/persistence/"
+    cond_redirect rsync --one-file-system --delete -aKh "/var/log/" "${syncMount}/var/log/"
+    # remove zram state captured from the live system, else zram-config skips device creation on the clone's first boot
+    rm -f "${syncMount}/usr/local/lib/zram-config/zram-device-list"
     if ! (umount "$syncMount" &> /dev/null); then
       sleep 1
       umount -l "$syncMount" &> /dev/null