diff --git a/.gitignore b/.gitignore
index 27d84c9..f678f57 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,8 +4,12 @@
 # User configuration
 # /config/
 /.keydb/
+/.spool/
+/.sessions.dets
 /config/prod*
 /config/runtime.exs
+/mock_spool/
+/mock_sessions.dets
 
 # If you run "mix test --cover", coverage assets end up here.
 /cover/
diff --git a/README.md b/README.md
index 445cf45..79a8d57 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,7 @@ if config_env() == :prod do
   config :floof, listen_opts: [ip: {192,168,0,1}]
   config :floof, pubkey_config: %{:keydb => ".keydb"}
   config :floof, spool_dir: ".spool"
+  config :floof, sessions_file: ".sessions.dets"
 end
 ```
 
@@ -49,6 +50,7 @@ if config_env() == :prod do
   config :floof, listen_port: nil
   config :floof, pubkey_config: %{:keydb => ".keydb"}
   config :floof, spool_dir: ".spool"
+  config :floof, sessions_file: ".sessions.dets"
 
   config :floof,
     upstreams: [
diff --git a/lib/floof.ex b/lib/floof.ex
index b4edb05..4a1d05a 100644
--- a/lib/floof.ex
+++ b/lib/floof.ex
@@ -82,7 +82,8 @@ defmodule Floof do
           backlog =
             if sesskey != nil do
               # make sure that stuff gets handled uniformly
-              Floof.SessionManager.set(sesskey, dcdhash, dcd)
+              Floof.SessionManager.set_soft(sesskey, dcdhash)
+              Floof.PacketSpool.store_new(dcdhash, dcd)
             else
               :ok = send_summary(client, :requestpull, [dcdhash])
               Map.put(backlog, dcdhash, dcd)
@@ -100,7 +101,10 @@ defmodule Floof do
         {:tcp_closed, _} ->
           # put all the backlogged stuff back if possible
           if sesskey != nil do
-            Floof.SessionManager.set_multi(sesskey, backlog)
+            for {dcdhash, dcd} <- backlog do
+              Floof.SessionManager.set_soft(sesskey, dcdhash)
+              Floof.PacketSpool.store_new(dcdhash, dcd)
+            end
             Floof.SessionManager.detach(sesskey, self())
           end
 
@@ -120,8 +124,11 @@ defmodule Floof do
     case dcd do
       {:session, {:attach, key}} ->
         Floof.SessionManager.attach(key, self())
-        Floof.SessionManager.set_multi(sesskey, backlog)
-        backlog
+        for {dcdhash, dcd} <- backlog do
+          Floof.SessionManager.set_soft(sesskey, dcdhash)
+          Floof.PacketSpool.store_new(dcdhash, dcd)
+        end
+        %{}
 
       {:summary, {:Summary, direction, hashes}} ->
         case direction do
@@ -136,7 +143,10 @@ defmodule Floof do
               item =
                 case Map.fetch(backlog, mhash) do
                   {:ok, item} -> item
-                  :error -> Floof.SessionManager.get(sesskey, mhash)
+                  :error -> case Floof.PacketSpool.fetch(mhash) do
+                    {:ok, item} -> item
+                    _ -> nil
+                  end
                 end
 
               if item != nil do
diff --git a/lib/floof/application.ex b/lib/floof/application.ex
index 13505e6..2951af7 100644
--- a/lib/floof/application.ex
+++ b/lib/floof/application.ex
@@ -17,6 +17,7 @@ defmodule Floof.Application do
     pubkey_mgr = Application.fetch_env!(:floof, :pubkey_mgr)
     pubkey_config = Application.fetch_env!(:floof, :pubkey_config)
     spool_dir = Application.fetch_env!(:floof, :spool_dir)
+    sessions_file = Application.fetch_env!(:floof, :sessions_file)
 
     children =
       [
@@ -33,7 +34,7 @@ defmodule Floof.Application do
            pubkey_mgr: pubkey_mgr,
            pubkey_config: pubkey_config
          )},
-        {Floof.SessionManager, %{}}
+        {Floof.SessionManager, {sessions_file, []}}
       ] ++
         if listen_port != nil do
           [{Task, fn -> Floof.accept(listen_port, listen_opts) end}]
diff --git a/lib/floof/packet_spool.ex b/lib/floof/packet_spool.ex
index 0e93062..a2d30b7 100644
--- a/lib/floof/packet_spool.ex
+++ b/lib/floof/packet_spool.ex
@@ -18,7 +18,11 @@ defmodule Floof.PacketSpool do
 
   ## packet is expected to be of type :XferBlob
   def store(hash, packet) do
-    GenServer.call(__MODULE__, {:store, hash, packet})
+    GenServer.call(__MODULE__, {:store, hash, packet, true})
+  end
+
+  def store_new(hash, packet) do
+    GenServer.call(__MODULE__, {:store, hash, packet, false})
   end
 
   def fetch(hash) do
@@ -29,6 +33,10 @@ defmodule Floof.PacketSpool do
     GenServer.cast(__MODULE__, {:drop, hash})
   end
 
+  def keep_only(hashes) do
+    GenServer.cast(__MODULE__, {:keep_only, hashes})
+  end
+
   ### server interface
 
   @impl true
@@ -37,11 +45,18 @@ defmodule Floof.PacketSpool do
   end
 
   @impl true
-  def handle_call({:store, hash, packet}, _, state) do
+  def handle_call({:store, hash, packet, overwrite}, _, state) do
+    path = hash_to_path(hash, state)
     echain(
       state,
-      fn -> :FloofProtocol.encode(:XferBlob, packet) end,
-      &File.write(hash_to_path(hash, state), &1)
+      fn ->
+        if not overwrite and File.exists?(path) do
+          :ok
+        else
+          :FloofProtocol.encode(:XferBlob, packet)
+        end
+      end,
+      &File.write(path, &1)
     )
   end
 
@@ -63,6 +78,33 @@ defmodule Floof.PacketSpool do
     {:noreply, state}
   end
 
+  @impl true
+  def handle_cast({:keep_only, hashes}, state) do
+    case File.ls(state) do
+      {:error, e} ->
+        Logger.error("spool: unable to browse spool directory for garbage collection: #{inspect(e)}")
+
+      {:ok, items} ->
+        present = MapSet.new(Enum.flat_map(items, fn x ->
+          case Base.url_decode64(x) do
+            {:ok, y} -> [y]
+            {:error, e} ->
+              Logger.warn("spool: unable to decode entry name #{x}: #{inspect(e)}")
+              []
+          end
+        end))
+        hashes = MapSet.new(hashes)
+        Logger.debug("spool: present #{inspect(present)} vs. to_keep #{inspect(hashes)}")
+        for hash <- MapSet.difference(present, hashes) do
+          case File.rm(hash_to_path(hash, state)) do
+            :ok -> nil
+            {:error, e} -> Logger.error("spool: unable to remove #{inspect(hash)}: #{inspect(e)}")
+          end
+        end
+    end
+    {:noreply, state}
+  end
+
   # internal interface
 
   defp hash_to_path(hash, state) do
diff --git a/lib/floof/session_manager.ex b/lib/floof/session_manager.ex
index 0135e2e..6767e4e 100644
--- a/lib/floof/session_manager.ex
+++ b/lib/floof/session_manager.ex
@@ -3,17 +3,20 @@ defmodule Floof.SessionManager do
   require Logger
   require Record
 
-  # structure of data
-  # {hash=>packet storage, sesskey=>MapSet(hashes) dets data}
+  # structure of data := %{dets file, subscription Map}
+  # dets data := {sesskey, hash} (via bag)
 
-  Record.defrecord(:sessent, sesskey: "", hashes: MapSet.new())
+  defstruct dets_file: "/var/spool/floof/sess.dets", subs: %{}
 
-  def start_link(initial_value) do
-    Agent.start_link(fn -> initial_value end, name: __MODULE__)
+  def start_link({file, opts}) do
+    Agent.start_link(fn ->
+      {:ok, file} = :dets.open_file(file, opts ++ [type: :bag])
+      %Floof.SessionManager{dets_file: file}
+    end, name: __MODULE__)
   end
 
   def attach(key, pid) do
-    modify_subscription(key, fn oldpid, m ->
+    modify_subscription(key, fn oldpid, dets_file ->
       if oldpid != pid do
         Floof.Distributor.unregister(pid)
 
@@ -23,8 +26,10 @@ defmodule Floof.SessionManager do
         end
       end
 
-      if not Enum.empty?(m) do
-        send(pid, {:SessionPushed, key})
+      case :dets.lookup(dets_file, key) do
+        [] -> nil
+        {:error, _} -> nil
+        _ -> send(pid, {:SessionPushed, key})
       end
 
       pid
@@ -44,114 +49,72 @@ defmodule Floof.SessionManager do
 
   defp modify_subscription(key, handler) do
     Agent.cast(__MODULE__, fn state ->
-      {_, state2} =
-        Map.get_and_update(state, key, fn ent ->
-          {oldpid, m} = entry_dfl(ent)
-          {nil, {handler.(oldpid, m), m}}
+      {_, subs2} =
+        Map.get_and_update(state.subs, key, fn oldpid ->
+          {nil, handler.(oldpid, state.dets_file)}
         end)
 
-      state2
+      %Floof.SessionManager{state | subs: subs2}
     end)
   end
 
-  def set(key, subkey, value) do
-    set_one_internal(key, &Map.put(&1, subkey, value))
-  end
-
-  def set_multi(_, %{}) do
-    nil
-  end
-
-  def set_multi(key, backlog) do
-    set_one_internal(key, &Map.merge(&1, backlog))
-  end
-
   def set_for_all(subkey, value, origin) do
+    Floof.PacketSpool.store(subkey, value)
     Agent.cast(__MODULE__, fn state ->
-      for {key, {sub, m}} <- state, into: %{} do
-        m2 =
-          if origin != sub do
-            if sub != nil do
-              send(sub, {:SessionPushed, key})
-            end
+      filter_keys = MapSet.new(for {key, sub} <- state.subs do
+        if origin != sub and sub != nil do
+          send(sub, {:SessionPushed, key, subkey})
+          key
+        else
+          nil
+        end
+      end)
 
-            Map.put(m, subkey, value)
-          else
-            m
-          end
+      {:ok, all_keys} = all_session_keys(state)
+      added_dets_ents = for key <- MapSet.difference(all_keys, filter_keys), do: {key, subkey}
+      :ok = :dets.insert(state.dets_file, added_dets_ents)
 
-        {key, {sub, m2}}
-      end
+      state
+    end)
+  end
+
+  def set_soft(key, subkey) do
+    Agent.cast(__MODULE__, fn state ->
+      :ok = :dets.insert(state.dets_file, {key, subkey})
+      state
     end)
   end
 
   def peek(key) do
     Agent.get(
       __MODULE__,
-      &MapSet.new(
-        case Map.get(&1, key) do
-          nil -> []
-          {_, m} -> Map.keys(m)
-        end
-      )
-    )
-  end
-
-  def get(key, subkey) do
-    Agent.get(
-      __MODULE__,
-      &case Map.get(&1, key) do
-        nil -> nil
-        {_, m} -> Map.get(m, subkey)
+      &case :dets.lookup(&1.dets_file, key) do
+        {:error, _} -> []
+        items -> Enum.map(items, fn {_, x} -> x end)
       end
     )
   end
 
-  def delete(key, subkey) do
-    xdrop_internal(key, &Map.delete(&1, subkey))
-  end
-
   def drop(key, subkeys) do
-    xdrop_internal(key, &Map.drop(&1, subkeys))
+    Agent.cast(__MODULE__, fn state ->
+      dets_file = state.dets_file
+      :ok = :dets.delete_object(dets_file, Enum.map(subkeys, fn x -> {key, x} end))
+
+      # garbage collect unused objects
+      case :dets.match(dets_file, {'_', '$1'}) do
+        {:error, e} ->
+          Logger.error("garbage collection error: #{inspect(e)}")
+
+        keys ->
+          Floof.PacketSpool.keep_only(for [key] <- keys, do: key)
+      end
+    end)
   end
 
-  defp entry_dfl(got) do
-    case got do
-      nil -> {nil, %{}}
-      x -> x
+  defp all_session_keys(state) do
+    case :dets.select(state.dets_file, [{{'_', '_'}, [], ['$1']}]) do
+      {:error, x} -> {:error, x}
+      items -> {:ok, Enum.map(items, fn {x} -> x end)}
     end
   end
-
-  defp set_one_internal(key, handler) do
-    Agent.cast(__MODULE__, fn state ->
-      {sub, state2} =
-        Map.get_and_update(state, key, fn ent ->
-          {sub, m} = entry_dfl(ent)
-          {sub, {sub, handler.(m)}}
-        end)
-
-      if sub != nil do
-        send(sub, {:SessionPushed, key})
-      end
-
-      state2
-    end)
-  end
-
-  defp xdrop_internal(key, handler) do
-    Agent.cast(__MODULE__, fn state ->
-      {_, m} =
-        Map.get_and_update(state, key, fn ent ->
-          {sub, m} = entry_dfl(ent)
-
-          case {sub, handler.(m)} do
-            {nil, %{}} -> :pop
-            x -> {nil, x}
-          end
-        end)
-
-      m
-    end)
-  end
 end
-
diff --git a/mix.exs b/mix.exs
index f4ad846..2e6ab68 100644
--- a/mix.exs
+++ b/mix.exs
@@ -35,8 +35,13 @@ defmodule Floof.MixProject do
         # which get added to all processed messages
         set_markers: [],
         pubkey_mgr: Floof.PubkeyMgr.FileDB,
+        # file locations
+        #  the keydb is only read, and is used to check message signatures
         pubkey_config: %{:keydb => "mock_keydb"},
-        spool_dir: "mock_spool"
+        #  the spool dir saves messages in-transit, and is used to prevent RAM OOM
+        spool_dir: "mock_spool",
+        #  the session file is used for persistence of session data across restarts
+        session_file: "mock_sessions.dets",
       ]
     ]
   end