fix(btrfs): allocate the swapfile with fallocate instead of dd

On a btrfs root the swapfile was written with dd from /dev/zero, so a
swapfile the size of the RAM cost tens of GiB of writes and stalled
every other writer on the filesystem. A caller that bounds the run, such
as an async Ansible task, killed dd halfway, and the next run found the
wrong size and started over. btrfs accepts a preallocated NOCOW
swapfile and the swap directory already carries +C, so every filesystem
now allocates with fallocate.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -27,10 +27,10 @@ flowchart TD
     I -- yes --> J["swapoff, delete it, drop its fstab line"]
     I -- no --> K
     J --> K{"root filesystem is btrfs?"}
-    K -- yes --> L["chattr +C, turn compression off, allocate with dd"]
+    K -- yes --> L["chattr +C, turn compression off"]
     K -- no --> M["allocate with fallocate"]
-    L --> N["chmod 600"]
+    L --> M
-    M --> N
+    M --> N["chmod 600"]
     N --> O["mkswap"]
     O --> P["swapon"]
     P --> Q["append the entry to /etc/fstab"]
@@ -43,7 +43,7 @@ The fstab entry is written **after** activation succeeds, so a failed run never
 ## 🛠 Features
 
 - **Automated Swapfile Creation:** Easily create a swapfile with a specified size.
-- **Btrfs Aware:** Disables copy-on-write and compression on the swap directory and allocates with `dd`; other filesystems use `fallocate`.
+- **Btrfs Aware:** Disables copy-on-write and compression on the swap directory before allocating; every filesystem allocates with `fallocate`.
 - **FSTAB Integration:** Automatically updates `/etc/fstab` to ensure the swapfile is mounted at boot.
 - **Safety Checks:** Skips creation when a swapfile of the correct size already exists, and recreates it when the size differs.
 - **Simple CLI Interface:** Run the command with a single argument.
@@ -66,9 +66,8 @@ The package installs **two** identical commands: `swap-forge` (primary) and `swa
 
 - **Python 3.10+** 🐍
 - **root privileges** — the tool writes `/etc/fstab` and activates swap
-- Always required on `PATH`: `findmnt`, `swapoff`, `chmod`, `mkswap`, `swapon`
+- Always required on `PATH`: `findmnt`, `swapoff`, `fallocate`, `chmod`, `mkswap`, `swapon`
-- On a **btrfs** root additionally: `chattr`, `btrfs`, `dd`
+- On a **btrfs** root additionally: `chattr`, `btrfs`
-- On any **other** root filesystem additionally: `fallocate`
 
 If a required command is missing, the tool exits with code `127` and a one‑line error instead of a traceback.
 

src/swap_forge/core.py

@@ -10,9 +10,8 @@ DEFAULT_SWAP_PATH = Path("/swapfile")
 SIZE_PATTERN = re.compile(r"^(\d+)([gGmM]?[bB]?)?$")
 MIB_PER_GIB = 1024
 
-REQUIRED_BINARIES = ("findmnt", "swapoff", "chmod", "mkswap", "swapon")
+REQUIRED_BINARIES = ("findmnt", "swapoff", "fallocate", "chmod", "mkswap", "swapon")
-BTRFS_BINARIES = ("chattr", "btrfs", "dd")
+BTRFS_BINARIES = ("chattr", "btrfs")
-NON_BTRFS_BINARIES = ("fallocate",)
 EXIT_MISSING_BINARY = 127
 EXIT_COMMAND_FAILED = 1
 
@@ -117,8 +116,8 @@ def create_swap(path, size_mib, fs, fstab=FSTAB_FILE):
         fs: Filesystem type of the root mount.
         fstab: Path of the fstab file to append to.
     """
+    print(f"Creating {size_mib} MiB swapfile on {fs} at {path}")
     if fs == "btrfs":
-        print(f"Creating {size_mib} MiB swapfile on btrfs at {path}")
         path.parent.mkdir(parents=True, exist_ok=True)
         run(["chattr", "+C", str(path.parent)], check=False)
         run(
@@ -133,20 +132,8 @@ def create_swap(path, size_mib, fs, fstab=FSTAB_FILE):
             ],
             check=False,
         )
-        run(
-            [
-                "dd",
-                "if=/dev/zero",
-                f"of={path}",
-                "bs=1M",
-                f"count={size_mib}",
-                "status=progress",
-            ]
-        )
-    else:
-        print(f"Creating {size_mib} MiB swapfile on {fs} at {path}")
-        run(["fallocate", "-l", f"{size_mib}M", str(path)])
 
+    run(["fallocate", "-l", f"{size_mib}M", str(path)])
     run(["chmod", "600", str(path)])
     run(["mkswap", str(path)])
     run(["swapon", str(path)])

tests/e2e/run.sh

@@ -21,7 +21,7 @@ case "${SWAPFORGE}" in
 esac
 echo "ok: resolved from the installed package, not the source tree"
 
-for binary in findmnt swapoff chmod mkswap swapon fallocate chattr btrfs dd; do
+for binary in findmnt swapoff fallocate chmod mkswap swapon chattr btrfs; do
     command -v "${binary}" >/dev/null || fail "${binary} is missing from the image"
 done
 echo "ok: every declared binary is present"

tests/unit/test_core.py

@@ -7,7 +7,6 @@ from swap_forge.core import (
     BTRFS_BINARIES,
     BTRFS_SWAP_PATH,
     DEFAULT_SWAP_PATH,
-    NON_BTRFS_BINARIES,
     REQUIRED_BINARIES,
     MissingBinaryError,
     create_swap,
@@ -22,28 +21,24 @@ from swap_forge.core import (
 
 class TestRequiredBinaries(unittest.TestCase):
     def test_declares_every_external_command_the_package_calls(self):
-        declared = (
+        declared = set(REQUIRED_BINARIES) | set(BTRFS_BINARIES)
-            set(REQUIRED_BINARIES) | set(BTRFS_BINARIES) | set(NON_BTRFS_BINARIES)
-        )
 
         self.assertEqual(
             declared,
             {
                 "findmnt",
                 "swapoff",
+                "fallocate",
                 "chmod",
                 "mkswap",
                 "swapon",
                 "chattr",
                 "btrfs",
-                "dd",
-                "fallocate",
             },
         )
 
     def test_filesystem_specific_binaries_are_not_declared_as_always_required(self):
         self.assertFalse(set(REQUIRED_BINARIES) & set(BTRFS_BINARIES))
-        self.assertFalse(set(REQUIRED_BINARIES) & set(NON_BTRFS_BINARIES))
 
     def test_missing_binary_is_reported_with_its_name(self):
         with mock.patch("swap_forge.core.subprocess.run") as subprocess_run:
@@ -153,7 +148,7 @@ class TestCreateSwap(unittest.TestCase):
             self.assertEqual(commands, ["fallocate", "chmod", "mkswap", "swapon"])
             self.assertEqual(fstab.read_text(), f"{swapfile} none swap sw 0 0\n")
 
-    def test_btrfs_disables_copy_on_write_and_uses_dd(self):
+    def test_btrfs_disables_copy_on_write_before_fallocate(self):
         with tempfile.TemporaryDirectory() as base:
             swapfile = Path(base) / "swap" / "swapfile"
             fstab = Path(base) / "fstab"
@@ -164,7 +159,11 @@ class TestCreateSwap(unittest.TestCase):
 
             commands = [call.args[0][0] for call in run.call_args_list]
             self.assertEqual(
-                commands, ["chattr", "btrfs", "dd", "chmod", "mkswap", "swapon"]
+                commands, ["chattr", "btrfs", "fallocate", "chmod", "mkswap", "swapon"]
+            )
+            self.assertEqual(
+                run.call_args_list[2].args[0],
+                ["fallocate", "-l", "1024M", str(swapfile)],
             )
             self.assertTrue(swapfile.parent.is_dir())