From d6f736cd831c34d0c2286412b9540cb7f676c445 Mon Sep 17 00:00:00 2001
From: "re:fi.64" <hello@refi64.dev>
Date: Tue, 21 Jul 2026 20:43:19 -0500
Subject: [PATCH] nix/spindle: fix almalinux10 images having an undersized
 rootfs

systemd-volatile-root uses only 25% of RAM for the rootfs, which is
below the tmpfs default of 50% and makes it hard to do anything useful.
There is no way to change this, so instead, we use a simple custom
script to set up the rootfs before the initrd exits, with a better size
used.
---
 nix/pkgs/spindle-almalinux-image.nix | 43 ++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/nix/pkgs/spindle-almalinux-image.nix b/nix/pkgs/spindle-almalinux-image.nix
index 7ef9f115..98a1ff5c 100644
--- a/nix/pkgs/spindle-almalinux-image.nix
+++ b/nix/pkgs/spindle-almalinux-image.nix
@@ -75,6 +75,41 @@
     ext4
   '';
 
+  # systemd-volatile-root defaults the overlay tmpfs size to 25% of RAM, which
+  # is smaller than the 50% default, too small to do anything useful
+  # (25% of 4GiB -> 1GiB, vs 50% -> 2GiB), and can't currently be customized:
+  # https://github.com/systemd/systemd/issues/25976
+  # instead, we create our own spindle-volatile-root service for the initrd that
+  # does the same thing but using the default tmpfs size
+  volatileRootScript = writeText "spindle-volatile-root.sh" ''
+    #!/bin/bash
+    set -ex
+
+    statedir=/run/spindle-volatile-root
+    mkdir -p $statedir
+    mount -t tmpfs tmpfs $statedir -o mode=0755
+
+    mkdir -p $statedir/{upper,work}
+    mount -t overlay overlay /sysroot -o lowerdir=/sysroot,upperdir=$statedir/upper,workdir=$statedir/work
+
+    umount $statedir
+    rm -d $statedir  # use rm, because rmdir isn't in the default initramfs
+  '';
+
+  volatileRootService = writeText "spindle-volatile-root.service" ''
+    [Unit]
+    Description=Spindle Volatile Root
+    DefaultDependencies=no
+    Conflicts=shutdown.target
+    After=sysroot.mount sysroot-usr.mount
+    Before=initrd-root-fs.target shutdown.target
+
+    [Service]
+    Type=oneshot
+    RemainAfterExit=yes
+    ExecStart=/usr/bin/spindle-volatile-root
+  '';
+
   initService = writeText "spindle-init.service" ''
     [Unit]
     Description=Spindle system initialization
@@ -142,8 +177,6 @@
 
         "root=/dev/vda"
         "rootfstype=squashfs"
-        # volatile rootfs
-        "systemd.volatile=overlay"
         # selinux is not particularly useful for microvm CI
         "selinux=0"
 
@@ -227,6 +260,12 @@ in
     # place the modules-load.d configuration in the initramfs, because
     # systemd-modules-load does not exist in the default rootfs
     install -D -m 0644 ${modulesConf} initrd-tree/etc/modules-load.d/spindle-modules.conf
+    # add the custom volatile root service
+    install -D -m 0755 ${volatileRootScript} initrd-tree/usr/bin/spindle-volatile-root
+    install -D -m 0755 ${volatileRootService} initrd-tree/usr/lib/systemd/system/spindle-volatile-root.service
+    mkdir -p initrd-tree/usr/lib/systemd/system/initrd-root-fs.target.requires
+    ln -s ../spindle-volatile-root.service initrd-tree/usr/lib/systemd/system/initrd-root-fs.target.requires/
+    # build the initrd
     (cd initrd-tree && find . | bsdtar --format=newc --uid=0 --gid=0 -cnf - -T -) \
       | zstd -12 > initrd
 
-- 
2.51.2