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diff --git a/installer/lib/raid.sh b/installer/lib/raid.sh
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+++ b/installer/lib/raid.sh
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+#!/usr/bin/env bash
+# raid.sh - Pure RAID-level logic (testable, no I/O).
+# Source after common.sh.
+
+#############################
+# Valid-level enumeration
+#############################
+
+# Print valid RAID levels for a given disk count, one per line.
+# Count <2: nothing printed (single disk = no RAID).
+# Count 2:  mirror, stripe
+# Count 3+: + raidz1
+# Count 4+: + raidz2
+# Count 5+: + raidz3
+raid_valid_levels_for_count() {
+    local count=$1
+    [[ $count -lt 2 ]] && return 0
+    echo mirror
+    echo stripe
+    [[ $count -ge 3 ]] && echo raidz1
+    [[ $count -ge 4 ]] && echo raidz2
+    [[ $count -ge 5 ]] && echo raidz3
+    return 0
+}
+
+# Return 0 if level is valid for the given disk count, 1 otherwise.
+# Empty level with count 1 is valid (no RAID).
+raid_is_valid() {
+    local level=$1 count=$2
+    if [[ $count -le 1 ]]; then
+        [[ -z "$level" ]]
+        return
+    fi
+    raid_valid_levels_for_count "$count" | grep -qxF "$level"
+}
+
+#############################
+# Usable-space computation
+#############################
+
+# Print usable bytes for a level given disk count, smallest-disk bytes,
+# and total bytes across all disks. Writes nothing and returns 1 for
+# unknown levels.
+#
+# Usage: raid_usable_bytes LEVEL COUNT SMALLEST_BYTES TOTAL_BYTES
+raid_usable_bytes() {
+    local level=$1 count=$2 smallest=$3 total=$4
+    case "$level" in
+        mirror)  echo "$smallest" ;;
+        stripe)  echo "$total" ;;
+        raidz1)  echo $(( (count - 1) * smallest )) ;;
+        raidz2)  echo $(( (count - 2) * smallest )) ;;
+        raidz3)  echo $(( (count - 3) * smallest )) ;;
+        *)       return 1 ;;
+    esac
+}
+
+# Print fault-tolerance (max number of disks that can fail) for a level
+# at the given disk count. Unknown level → return 1.
+raid_fault_tolerance() {
+    local level=$1 count=$2
+    case "$level" in
+        mirror)  echo $(( count - 1 )) ;;
+        stripe)  echo 0 ;;
+        raidz1)  echo 1 ;;
+        raidz2)  echo 2 ;;
+        raidz3)  echo 3 ;;
+        *)       return 1 ;;
+    esac
+}