cluster/nix/modules/ceph.nix

# This runs Ceph on hscloud cluster(s).
#
# This lightly wraps the upstream NixOS ceph module, which is already fairly light.
#
# Most importantly, it does _not_ attempt to do any cluster
# bootstrapping/maintenance. This means, that any configuration action that
# does the following:
#  0. Bringing up a cluster
#  1. Adding/removing Mons
#  2. Changing a Mon IP address
#  3. Adding/removing OSDs
# ... must be done in tandem with manual operations on the affected nodes. For
# example, bootstrapping a cluster will involve keychain and monmap management,
# changing anything with mons will involve monmap management, adding new OSDs
# will require provisioning them with ceph-volume, etc.
#
# This is in stark contrast to a fully-managed solution like rook. Since we
# don't have hundreds of clusters, none of the above is automated, especially
# as that kind of automation is quite tricky to do reliably.

{ config, lib, pkgs, ... }:

with builtins;
with lib;

with (( import ../defs-cluster-k0.nix ) config.networking.hostName);

let
  machineName = config.networking.hostName;
  isMon = hasAttr machineName cephCluster.mons;
  isOsd = hasAttr machineName cephCluster.osds;
  hasCeph = isMon || isOsd;

  # This NixOS Ceph option fragment is present on every machine that runs a
  # mon, and basically tells the NixOS machinery to run mons/mgrs if needed on
  # this machine.
  cephMonConfig = if isMon then {
    mon = {
      enable = true;
      daemons = [ machineName ];
    };
    mgr = {
      enable = true;
      daemons = [ machineName ];
    };
  } else {};

  # Same as for cephMonConfig, but this time for OSDs.
  cephOsdConfig = if isOsd then {
    osd = {
      enable = true;
      daemons = map (el: "${toString el.id}") cephCluster.osds.${machineName};
    };
  } else {};

  # The full option fragment for services.ceph. It contains ceph.conf fragments
  # (in .global.*) and merges ceph{Mon,Osd}Config.
  cephConfig = {
    enable = true;
    global = {
      fsid = cephCluster.fsid;
      clusterName = cephCluster.name;

      # Every Ceph node always attempts to connect to all mons.
      monHost = concatStringsSep "," (mapAttrsToList (k: _: machinesByName.${k}.ipAddr) cephCluster.mons);
      monInitialMembers = concatStringsSep "," (builtins.attrNames cephCluster.mons);
    };
  } // cephMonConfig // cephOsdConfig;

  # Merge ceph-volume lvm activate into ceph-osd-ID services.
  #
  # This is because the upstream module seems to have been written with
  # filestore in mind, not bluestore. Filestore is relatively simple: an xfs
  # filesystem is mounted into /var/lib/caph/osd/$cluster-$id, that in turn
  # contains everything for that OSD to work. 
  #
  # Bluestore is a bit different. Instead of a normal filesystem being mounted,
  # Ceph manages a block device fully using LVM (and in our case, dmcrypt).
  # Every bluestore volume needs to be 'activated' before it can be used by an
  # OSD. Activation takes care of doing LVM and dmcrypt mounts, and prepares
  # the /var/lib/ceph/osd/$cluster-$id directory as if a filestore was present
  # there. However, instead of this being a diskmount, it's instead a tmpfs
  # into which a bunch of files are dropped, loaded from the LVM raw device.
  #
  # To make the upstream NixOS module OSD work with bluestore, we do the following:
  #  1. Change ConditionPathExists from the OSD mount into a /dev/disk/by-id
  #     path. This gates the service on that device being present.
  #  2. Inject an ExecStartPre which runs ceph-volume lvm activate, if needed.
  #  3. Add lvm/cryptsetup to the PATH of the service (as used by ceph-volume,
  #     which seems to look for them on PATH instead of being properly
  #     nixified).
  #
  # We also inject smartmontools into PATH for smartctl, which allows the OSD
  # to monitor device health.
  osdActivateServices = listToAttrs (map (el: let
      osdId = toString el.id;
      osdUuid = el.uuid;
      diskPath = el.path;
    in {
    name = "ceph-osd-${osdId}";
    value = {
      path = with pkgs; [
        lvm2
        cryptsetup
        smartmontools
      ];
      serviceConfig = {
        ExecStartPre = lib.mkForce [
          ("+" + (toString (pkgs.writeScript "ceph-osd-${osdId}-activate.sh" ''
            #!/bin/sh
            set -e
            dir="/var/lib/ceph/osd/${cephCluster.name}-${osdId}/"
            disk="${el.path}"
            uuid="${osdUuid}"
            if [ -d "$dir" ] && [ -f "$dir"/keyring ]; then
              echo "Volume $dir already activated, skipping..."
            else
              echo "Activating $dir with $disk, uuid $uuid..."
              ${pkgs.ceph}/bin/ceph-volume lvm activate --bluestore --no-systemd ${osdId} $uuid
            fi

          '')))

          "${pkgs.ceph.lib}/libexec/ceph/ceph-osd-prestart.sh --id ${osdId} --cluster ${cephCluster.name}"
        ];
      };
      unitConfig = {
        ConditionPathExists = lib.mkForce el.path;
      };
    };
  }) (if isOsd then cephCluster.osds.${machineName} else []));

in rec {
  services.ceph = if hasCeph then cephConfig else {};

  environment.systemPackages = with pkgs; [
    ceph cryptsetup smartmontools
  ];

  systemd.services = osdActivateServices;

  # Hack - the upstream ceph module should generate ${clusterName}.conf instead
  # of ceph.conf, let's just symlink it.
  environment.etc."ceph/${cephCluster.name}.conf".source = "/etc/ceph/ceph.conf";
}