Overriding memory allocation for build VM for OCIImage

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1

I’m trying to build an Oracle Cloud image as one of the outputs of my flake. I have an Oracle ARM VM, and my local machine is x86-based. I configured the output thus:

        oci-base = nixpkgs.lib.nixosSystem {
          specialArgs = {
            inherit inputs;
          };
          system = "aarch64-linux";
          modules = [
            "${nixpkgs}/nixos/modules/virtualisation/oci-image.nix"
            ./configuration-oracle.nix
          ];
        };

and added

  boot.binfmt.emulatedSystems = [ "aarch64-linux" ];

When I try to build the output, I get an out-of-memory error coming from the build VM.

 ➜  nix build .#nixosConfigurations.oci-base.config.system.build.OCIImage
...
       > error while reading directory /build/root/nix/store: Cannot allocate memory
...

along with many similar errors.

I see that under the hood, the OCIImage attribute is defined by reference to make-disk-image.nix, which does have a parameter memSize. But I cannot figure out how to override OCIImage appropriately to set memSize. I have a lot of experience as a Nix user but not a lot with the Nix language. Guidance much appreciated.

1 Like
2

I have encountered something similar. I even tried setting memSize to something much larger and it did not help, so I suspect something else is at play preventing these x86_64-linux→aarch64-linux cross builds. Here is the flake I used:

{
  description = "Minimal NixOS qcow2 image";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs }:
    let
      system = "aarch64-linux";
      nixos = self.nixosConfigurations.default;
    in
    {
      nixosConfigurations.default = nixpkgs.lib.nixosSystem {
        inherit system;
        modules = [ ./configuration.nix ];
      };

      packages.${system}.default = import "${nixpkgs}/nixos/lib/make-disk-image.nix" {
        inherit (nixos) config pkgs;
        lib = nixpkgs.lib;
        inherit (nixos.config.virtualisation) diskSize;
        format = "qcow2";
        partitionTableType = "efi";
        memSize = 8192; # MiB of RAM for the build VM (default is 1024)
      };
    };
}

This is part of the log from the failed build:

copying staging root to image...
[    0.000000] Linux version 6.6.0 (nixbld@localhost) (gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.44) #1 Fri Jan 16 21:37:51 UTC 2026
[    0.000000] memblock address range: 0x7fffd5c00000 - 0x7fffdc000000
[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x00007fffd5c00000-0x00007fffdbffffff]
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x00007fffd5c00000-0x00007fffdbffffff]
[    0.000000] Initmem setup node 0 [mem 0x00007fffd5c00000-0x00007fffdbffffff]
[    0.000000] Kernel command line:  mem=100M virtio_mmio.device=352@0x1000000:1
[    0.000000] Dentry cache hash table entries: 16384 (order: 5, 131072 bytes, linear)
[    0.000000] Inode-cache hash table entries: 8192 (order: 4, 65536 bytes, linear)
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 25250
[    0.000000] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
[    0.000000] Memory: 100768K/102400K available (6905K kernel code, 1663K rwdata, 1557K rodata, 139K init, 337K bss, 1632K reserved, 0K cma-reserved)
[    0.000000] SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
[    0.000000] trace event string verifier disabled
[    0.000000] NR_IRQS: 4096
[    0.000000] lkl: irqs initialized
[    0.000000] clocksource: lkl: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
[    0.000022] lkl: time and timers initialized (irq2)
[    0.000780] pid_max: default: 4096 minimum: 301
[    0.003139] Mount-cache hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.003178] Mountpoint-cache hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.075203] printk: console [lkl_console0] enabled
[    0.076985] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns
[    0.079475] NET: Registered PF_NETLINK/PF_ROUTE protocol family
[    0.085502] lkl_pci: probe of lkl_pci failed with error -1
[    0.210197] raid6: skipped pq benchmark and selected int64x8
[    0.210321] raid6: using intx1 recovery algorithm
[    0.223308] vgaarb: loaded
[    0.224253] clocksource: Switched to clocksource lkl
[    0.231317] NET: Registered PF_INET protocol family
[    0.232960] IP idents hash table entries: 2048 (order: 2, 16384 bytes, linear)
[    0.237611] tcp_listen_portaddr_hash hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.238048] Table-perturb hash table entries: 65536 (order: 6, 262144 bytes, linear)
[    0.238165] TCP established hash table entries: 1024 (order: 1, 8192 bytes, linear)
[    0.238370] TCP bind hash table entries: 1024 (order: 2, 16384 bytes, linear)
[    0.238552] TCP: Hash tables configured (established 1024 bind 1024)
[    0.239868] UDP hash table entries: 128 (order: 0, 4096 bytes, linear)
[    0.240181] UDP-Lite hash table entries: 128 (order: 0, 4096 bytes, linear)
[    0.241579] PCI: CLS 0 bytes, default 32
[    0.245377] virtio-mmio: Registering device virtio-mmio.0 at 0x1000000-0x100015f, IRQ 1.
[    0.271972] workingset: timestamp_bits=62 max_order=15 bucket_order=0
[    0.274533] SGI XFS with ACLs, security attributes, no debug enabled
[    0.279721] xor: automatically using best checksumming function   8regs
[    0.280045] io scheduler mq-deadline registered
[    0.280161] io scheduler kyber registered
[    0.317852] virtio_blk virtio0: 1/0/0 default/read/poll queues
[    0.323520] virtio_blk virtio0: [vda] 16777216 512-byte logical blocks (8.59 GB/8.00 GiB)
[    0.341977]  vda: vda1 vda2
[    0.354603] NET: Registered PF_INET6 protocol family
[    0.363353] Segment Routing with IPv6
[    0.363571] In-situ OAM (IOAM) with IPv6
[    0.364043] sit: IPv6, IPv4 and MPLS over IPv4 tunneling driver
[    0.395284] Btrfs loaded, zoned=no, fsverity=no
[    0.400095] Warning: unable to open an initial console.
[    0.400579] This architecture does not have kernel memory protection.
[    0.400730] Run /init as init process
[    0.504159] EXT4-fs (vda2): mounted filesystem ecd795c1-b1c6-4763-8867-556d73376965 r/w with ordered data mode. Quota mode: disabled.
error while reading directory /build/root/etc: Cannot allocate memory
3

After more experimentation, my experience is the same as yours. I vendored the module from nixpkgs into my flake so I could set memSize, only to find it made no difference.
I don’t have the expertise with qemu to figure out what’s really going on.