fix: flush the image before importing a VM disk

docs/vm.md

@@ -70,7 +70,7 @@ layouts are understood and the algorithm follows from the hash length.
 
 | Function | Contract |
 | --- | --- |
-| `vm_import_disk <vmid> <image> <storage> [format]` | `qm disk import` (or `qm importdisk` on older hosts) with `DISK_IMPORT_FORMAT`; sets `VM_IMPORTED_DISK` to the volume Proxmox reported or the `unusedN` entry the import added. Never guesses a name. |
+| `vm_import_disk <vmid> <image> <storage> [format]` | Flushes the image to disk (`sync -f`, best effort), then `qm disk import` (or `qm importdisk` on older hosts) with `DISK_IMPORT_FORMAT`; sets `VM_IMPORTED_DISK` to the volume Proxmox reported or the `unusedN` entry the import added. Never guesses a name. |
 | `kvm_check` | Part of `vm_preflight`, as on Incus: exit 236 when `/dev/kvm` is missing, before any question or download. `qm start` refuses a KVM guest without it. |
 | `vm_fit_cores` | Cuts `CORE_COUNT` to the host's CPU threads, with a warning. `vm_start_script` calls it after `default_settings`; `vm_prompt_cpu_cores` refuses a larger answer. |
 | `vm_claim_vmid` | Call right before `qm create`: re-checks `VMID`, which was picked before the download. An auto-picked ID another run took moves to the next free one and the disk names follow; an ID chosen in advanced settings stops with exit 205. |

pve/vm-core.func

@@ -2630,6 +2630,12 @@ vm_resize_disk() {
 # entry the import added. Scripts used to guess the name or take the last
 # `pvesm list` line, which sorts disk-10 before disk-9. The format defaults to
 # DISK_IMPORT_FORMAT from vm_select_storage.
+#
+# The image is flushed to disk first. Images are often just extracted or
+# converted (xz, gzip, qemu-img, virt-customize), leaving gigabytes of dirty
+# pages. When they live on the same ZFS pool as the target (rpool/var-lib-vz
+# and rpool/data on a default install), the zvol create inside the import runs
+# into Proxmox's zfs command timeout ("got timeout") while ZFS writes them back.
 vm_import_disk() {
   local vmid="${1:?vmid}" file="${2:?image}" storage="${3:?storage}" format="${4:-${DISK_IMPORT_FORMAT:-}}"
   local out before volume
@@ -2640,6 +2646,10 @@ vm_import_disk() {
   [[ -n "$format" ]] && opts=(--format "$format")
   before="$(qm config "$vmid" 2>/dev/null | sed -n 's/^unused[0-9]*: //p')" || before=""
 
+  msg_info "Flushing $(basename "$file") to disk"
+  # Best effort: a failed flush is no reason to skip the import.
+  sync -f "$file" 2>/dev/null || sync 2>/dev/null || true
+
   msg_info "Importing $(basename "$file") to ${storage}"
   if ! out="$("${cmd[@]}" "$vmid" "$file" "$storage" "${opts[@]}" 2>&1)"; then
     msg_error "Importing $(basename "$file") into VM ${vmid} failed"

tests/vm-helpers.sh

@@ -237,6 +237,7 @@ qm() {
   *) return 0 ;;
   esac
 }
+sync() { printf 'sync %s\n' "$*" >>"$LOG"; return "${SYNC_RC:-0}"; }
 pct() { return 1; }
 lvs() { :; }
 pvesh() { printf '%s\n' "${NEXTID:-105}"; }
@@ -256,6 +257,21 @@ assert_eq "$VM_IMPORTED_DISK" "local-lvm:vm-100-disk-1" "volume from qm's report
 assert_contains "$LOG" "qm disk import 100 $IMAGE local-lvm --format raw"
 ok "vm_import_disk uses qm disk import and the reported volume"
 
+# Dirty pages from extraction stall the zvol create on the same ZFS pool (#18580).
+sync_line="$(grep -n "^sync -f $IMAGE\$" "$LOG" | cut -d: -f1)"
+import_line="$(grep -n "^qm disk import 100 " "$LOG" | cut -d: -f1)"
+[[ -n "$sync_line" && -n "$import_line" && "$sync_line" -lt "$import_line" ]] ||
+  fail "the image must be flushed before qm disk import"
+ok "vm_import_disk flushes the image before importing it"
+
+import_case
+SYNC_RC=1
+vm_import_disk 100 "$IMAGE" local-lvm || fail "a failed flush must not stop the import"
+assert_eq "$VM_IMPORTED_DISK" "local-lvm:vm-100-disk-1" "imported despite the failed flush"
+assert_contains "$LOG" "qm disk import 100 $IMAGE local-lvm"
+SYNC_RC=0
+ok "vm_import_disk imports even when the flush fails"
+
 import_case
 QM_MODERN=no
 vm_import_disk 100 "$IMAGE" local-lvm qcow2 || fail "legacy import"