Fix zvol-iscsi backup: fd conflict, case-insensitive zvol lookup, and snapdev handling

- Use fd 3 for domain list to avoid stdin conflicts with inner reads
- Resolve zvol names case-insensitively to match lowercased iSCSI IQNs
- Toggle snapdev=visible/hidden so snapshot block devices appear for dd
- Wait for snapshot device with udevadm settle before reading
- Pass volsize to pv for accurate progress display
- Document volmode=dev prerequisite to avoid zvol_create_minors deadlock

kvm-backup-zvol-iscsi.sh

@@ -5,6 +5,13 @@
 # Runs on the ZFS server where the zvols live. VM domains are
 # discovered via virsh on remote libvirt hosts connected over iSCSI,
 # matching the domain-driven approach of the other kvm-backup scripts.
+#
+# Prerequisites:
+#   Set volmode=dev on each zvol before running this script to prevent
+#   kernel partition scanning on snapshot devices. This must be done
+#   once per zvol (not toggled at runtime) to avoid a deadlock in
+#   zvol_create_minors when combined with snapdev toggling.
+#     zfs set volmode=dev tank/kvm/<name>
 
 # A file to prevent overlapping runs.
 PIDFILE="/tmp/backup-zvol-iscsi.pid"
@@ -133,7 +140,7 @@ backupDomains() {
     local HOST_LABEL="$2"
 
     DOMLIST_STATUS_TMP="/tmp/backup-zvol-domlist-$HOST_LABEL-tmp"
-    while read -r _ DOMAIN _; do
+    while read -r _ DOMAIN _ <&3; do
         # If the domain is empty, skip.
         if [[ -z "$DOMAIN" ]]; then
             continue
@@ -166,12 +173,21 @@ backupDomains() {
                 continue
             fi
 
-            # Verify the zvol exists locally.
-            ZVOL="$ZFS_PARENT/$ZVOL_NAME"
-            if ! zfs list "$ZVOL" &>/dev/null; then
-                echo "Warning: zvol $ZVOL does not exist locally, skipping"
+            # Resolve the zvol name case-insensitively, since iSCSI IQN
+            # target names are typically lowercased.
+            RESOLVED_NAME=$(zfs list -o name -H -r "$ZFS_PARENT" -d 1 | while read -r ZN; do
+                BN="${ZN##*/}"
+                if [[ "${BN,,}" == "${ZVOL_NAME,,}" ]]; then
+                    echo "$BN"
+                    break
+                fi
+            done)
+            if [[ -z "$RESOLVED_NAME" ]]; then
+                echo "Warning: zvol matching $ZFS_PARENT/$ZVOL_NAME does not exist locally, skipping"
                 continue
             fi
+            ZVOL_NAME="$RESOLVED_NAME"
+            ZVOL="$ZFS_PARENT/$ZVOL_NAME"
 
             DEVS+=("$DEV")
             ZVOL_NAMES+=("$ZVOL_NAME")
@@ -202,19 +218,33 @@ backupDomains() {
             ZVOL_NAME=${ZVOL_NAMES[$i]}
             ZVOL="$ZFS_PARENT/$ZVOL_NAME"
 
+            # Make snapshot block devices visible so we can read them.
+            zfs set snapdev=visible "$ZVOL"
+
             # Create a ZFS snapshot.
             echo "Creating snapshot: $ZVOL@$SNAPSHOT_NAME"
             if ! zfs snapshot "$ZVOL@$SNAPSHOT_NAME"; then
                 fail "Failed to create snapshot for $ZVOL"
             fi
 
+            # Wait for the snapshot block device to appear.
+            SNAP_DEV="/dev/zvol/$ZVOL@$SNAPSHOT_NAME"
+            udevadm settle
+            for _ in $(seq 1 30); do
+                [[ -e "$SNAP_DEV" ]] && break
+                sleep 1
+            done
+            if [[ ! -e "$SNAP_DEV" ]]; then
+                fail "Snapshot device $SNAP_DEV did not appear"
+            fi
+
             # Read the raw disk image from the snapshot's block device.
             # This produces a portable raw image that can be restored to
             # any virtual disk or converted with qemu-img, without
             # requiring ZFS on the restore target.
-            SNAP_DEV="/dev/zvol/$ZVOL@$SNAPSHOT_NAME"
+            ZVOL_SIZE=$(zfs list -Hp -o volsize "$ZVOL")
             echo "Creating backup for $DOMAIN ($DEV [$ZVOL])"
-            if ! dd if="$SNAP_DEV" bs=4M status=none | pv | borg create \
+            if ! dd if="$SNAP_DEV" bs=4M status=none | pv -s "$ZVOL_SIZE" | borg create \
                     --verbose \
                     --stats \
                     --show-rc \
@@ -233,6 +263,9 @@ backupDomains() {
                 fi
             fi
 
+            # Hide snapshot devices again.
+            zfs set snapdev=hidden "$ZVOL"
+
             # Cleanup old ZFS snapshots.
             cleanupSnapshots "$ZVOL"
         done
@@ -267,7 +300,7 @@ backupDomains() {
                 fail "Failed to prune $DOMAIN xml"
             fi
         fi
-    done < <(
+    done 3< <(
         if [[ -n "$SSH_PREFIX" ]]; then
             $SSH_PREFIX "virsh list --all" 2>/dev/null | tail -n +3
         else