Disable fsck for MergerFS?

[dinvlad]

First of all, thanks for the excellent piece of software!

I've configured MergerFS through Disko:

    mergerfs = {
        type = "filesystem";
        device = "/mnt/data_*";
        content = {
          type = "filesystem";
          format = "fuse.mergerfs";
          mountpoint = "/mnt/data";
          mountOptions = [
            "noatime"
            "defaults"
            "cache.files=partial"
            "dropcacheonclose=true"
            "category.create=mfs"
          ];
        };
      };
    };

(here, disk(s) already mounted to /mnt/data_1, /mnt/data_2 are then mounted as a combined FUSE filesystem to /mnt/data)

This works, but at mount time I see the following in dmesg:

[   22.376728] systemd-fstab-generator[1683]: Checking was requested for "/mnt/data_*", but it is not a device.

And fstab shows this:

➜  ~ grep '/mnt/data ' /etc/fstab
/mnt/data_* /mnt/data fuse.mergerfs noatime,defaults,cache.files=partial,dropcacheonclose=true,category.create=mfs 0 2

As you can see, fsck check is enabled due to the last flag (2).

Is it possible to disable it through Disko config somehow? Thank you!

[iFreilicht]

First of all, a really fun hack that uses the fact that we allow arbitrary strings in the format. AFAIU, you only used disko --mode mount or even just the module, right? Because I think --mode format would fail with this, as /mnt/data_* is not an actual device.

I think the "proper" solution for this issue would be for us to support mergerfs as an actual filesystem, with options dedicated to it, but as a workaround for your situation, it should be enough to add this top-level option to your config:

  fileSystems.mergerfs.noCheck = lib.mkForce true;

Just out of curiosity, did you have to do anything else to get mergerfs to work? Also, could you try if instead of "fuse.mergerfs", simply "mergerfs" works as well?

[dinvlad]

OK, thanks for your suggestions! I set lib.mkForce true and format = "mergerfs" successfully.

Other than that, the only other pieces I added earlier to make it work, in configuration.nix, are:

  environment.systemPackages = with pkgs; {
    mergerfs
    # ...
  };

  systemd.services.mergerfs-uncache = {
    path = [
      (pkgs.python3.withPackages (ps: with ps; [
        aiofiles
      ]))
    ];
    serviceConfig = {
      Type = "oneshot";
      ExecStart = ''
        ${./mergerfs-uncache.py} -s /mnt/cache -d /mnt/mergerfs_slow -a 90 -t 90
      '';
    };
    startAt = "Sat 00:00:00";
  };

where mergerfs-uncache.py is a slightly modified version of https://github.com/notthebee/nix-config/blob/main/modules/mover/mergerfs-uncache.py (I added -a, --atime option handling). This script is used to move more rarely accessed data from /mnt/cache (which is my fast NVME RAID 1 array) to /mnt/mergerfs_slow (HDD SnapRAID array).

Note also that since my last post, I've modified Disko config to use 2 nested MergerFS filesystems, such that I could access the combined NVME+HDD storage under /data (and under the hood, it would first write the data to NVMEs and then slowly over time move it to HDDs - thus achieving a tiered storage solution):

let
  cacheArray = "/mnt/cache";
  slowArray = "/mnt/data*";
  mergerArrays = {
    mergerfs_slow = {
      device = slowArray;
      mountpoint = "/mnt/mergerfs_slow";
      extraOpts = [];
    };
    data = {
      device = "${cacheArray}:${slowArray}";
      mountpoint = "/data";
      extraOpts = ["category.create=epff"];
    };
  };
  # ...
in
  { disks ? [], lib, ... }: {

  fileSystems = lib.mapAttrs' (_: fs: lib.nameValuePair fs.mountpoint {
    noCheck = lib.mkForce true;
  }) mergerArrays;

  disko.devices = {
    disk = (
      lib.mapAttrs (name: fs : {
        type = "filesystem";
        device = fs.device;
        content = {
          type = "filesystem";
          format = "fuse.mergerfs";
          mountpoint = fs.mountpoint;
          mountOptions = [
            "defaults"
            "moveonenospc=1"
            "minfreespace=100G"
            "func.getattr=newest"
            "cache.files=partial"
            "dropcacheonclose=true"
            "fsname=${name}"
          ] ++ fs.extraOpts;
        };
      }) mergerArrays
    ) // # ... the rest of Disko config, for the actual disks
};