diff --git a/.env.example b/.env.example
index ccd4b5a..ecbe9f5 100644
--- a/.env.example
+++ b/.env.example
@@ -59,8 +59,9 @@ VERSION=2022
 RAM_SIZE=8G
 CPU_CORES=4
 DISK_SIZE=100G
-# QEMU disk I/O backend: native (default, works on any kernel), threads, or
-# io_uring (faster, needs kernel 5.12 or newer; older kernels fail at boot).
+# QEMU disk I/O backend: native, threads or io_uring. Empty lets QEMU choose,
+# which is io_uring on recent kernels. Kernels before 5.12 (Ubuntu 20.04) fail
+# at boot with "Failed to initialize io_uring": set native there.
 # DISK_IO=native
 # Local administrator created by the unattended install; also the RDP login
 WIN_USERNAME=admin
diff --git a/README.md b/README.md
index df2d4b7..3d1c914 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@ Two authentication methods are supported. Set **one** of them in `.env`:
     | `INSTALL_RTOOLS` | `true` | Install Rtools 4.0 with `mingw-w64-x86_64-make` |
     | `VERSION` | `2022` | dockur Windows release (`2022` = Windows Server 2022 evaluation) |
     | `RAM_SIZE` / `CPU_CORES` / `DISK_SIZE` | `8G` / `4` / `100G` | VM size |
-| `DISK_IO` | `native` | QEMU disk I/O backend. `io_uring` is faster but needs kernel 5.12 or newer; on older kernels QEMU exits at boot with `Failed to initialize io_uring` |
+| `DISK_IO` | QEMU's choice | QEMU disk I/O backend: `native`, `threads` or `io_uring`. Kernels before 5.12 exit at boot with `Failed to initialize io_uring`; set `native` there |
     | `WIN_USERNAME` / `WIN_PASSWORD` | `admin` / `runner` | Local administrator created by the install, also the RDP login |
 
 2. Start it and follow the container side (ISO download, QEMU boot):
diff --git a/docker-compose.yml b/docker-compose.yml
index 48c363b..3857725 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -17,9 +17,9 @@ services:
       RAM_SIZE: "${RAM_SIZE:-8G}"
       CPU_CORES: "${CPU_CORES:-4}"
       DISK_SIZE: "${DISK_SIZE:-100G}"
-      # QEMU disk I/O backend. dockur picks io_uring, which fails on kernels
-      # before 5.12 with "Failed to initialize io_uring: Cannot allocate memory".
-      DISK_IO: "${DISK_IO:-native}"
+      # QEMU disk I/O backend; empty lets QEMU choose. Set DISK_IO=native in .env
+      # on kernels before 5.12 ("Failed to initialize io_uring" at boot).
+      DISK_IO: "${DISK_IO:-}"
       USERNAME: "${WIN_USERNAME:-admin}"
       PASSWORD: "${WIN_PASSWORD:-runner}"
     devices: