diff --git a/bench/Normal.hs b/bench/Normal.hs
--- a/bench/Normal.hs
+++ b/bench/Normal.hs
@@ -62,7 +62,7 @@
 
 normalBench :: GenIO -> IO ()
 normalBench g = do
-  let s = quiet $ noSave $ status "Normal" (const 1) lh proposals mon 0 nBurn nAutoTune nIter g
+  let s = quiet $ status "Normal" (const 1) lh proposals mon 0 nBurn nAutoTune nIter g
   void $ mh s
 
 proposalsBactrian :: Cycle Double
@@ -72,5 +72,5 @@
 
 normalBactrianBench :: GenIO -> IO ()
 normalBactrianBench g = do
-  let s = quiet $ noSave $ status "NormalBactrian" (const 1) lh proposalsBactrian mon 0 nBurn nAutoTune nIter g
+  let s = quiet $ status "NormalBactrian" (const 1) lh proposalsBactrian mon 0 nBurn nAutoTune nIter g
   void $ mh s
diff --git a/bench/Poisson.hs b/bench/Poisson.hs
--- a/bench/Poisson.hs
+++ b/bench/Poisson.hs
@@ -82,5 +82,5 @@
 
 poissonBench :: GenIO -> IO ()
 poissonBench g = do
-  let s = quiet $ noSave $ status "Poisson" (const 1) lh proposals mon initial nBurn nAutoTune nIter g
+  let s = quiet $ status "Poisson" (const 1) lh proposals mon initial nBurn nAutoTune nIter g
   void $ mh s
diff --git a/mcmc.cabal b/mcmc.cabal
--- a/mcmc.cabal
+++ b/mcmc.cabal
@@ -1,11 +1,6 @@
-cabal-version: 2.2
-
--- This file has been generated from package.yaml by hpack version 0.34.2.
---
--- see: https://github.com/sol/hpack
-
+cabal-version:  2.2
 name:           mcmc
-version:        0.2.1
+version:        0.2.2
 synopsis:       Sample from a posterior using Markov chain Monte Carlo
 description:    Please see the README on GitHub at <https://github.com/dschrempf/mcmc#readme>
 category:       Math, Statistics
@@ -16,6 +11,7 @@
 copyright:      Dominik Schrempf (2020)
 license:        GPL-3.0-or-later
 build-type:     Simple
+
 extra-source-files:
     README.md
     ChangeLog.md
@@ -55,6 +51,7 @@
   ghc-options: -Wall
   build-depends:
       aeson
+    , async
     , base >=4.7 && <5
     , bytestring
     , containers
diff --git a/src/Mcmc.hs b/src/Mcmc.hs
--- a/src/Mcmc.hs
+++ b/src/Mcmc.hs
@@ -110,7 +110,7 @@
     -- space (see above) and to monitor the MCMC run, as well as some auxiliary
     -- information.
     status,
-    noSave,
+    saveWith,
     force,
     quiet,
     debug,
diff --git a/src/Mcmc/Mcmc.hs b/src/Mcmc/Mcmc.hs
--- a/src/Mcmc/Mcmc.hs
+++ b/src/Mcmc/Mcmc.hs
@@ -196,12 +196,13 @@
 mcmcSave :: ToJSON a => Mcmc a ()
 mcmcSave = do
   s <- get
-  if save s
-    then do
-      mcmcInfoT "Save Markov chain. For long chains, this may take a while."
+  case save s of
+    Just n -> do
+      mcmcInfoT $ "Save Markov chain with trace of length " <> BL.pack (show n) <> "."
+      mcmcInfoT "For long traces, or complex objects, this may take a while."
       liftIO $ saveStatus (name s <> ".mcmc") s
       mcmcInfoT "Done saving Markov chain."
-    else mcmcInfoT "Do not save the Markov chain."
+    Nothing -> mcmcInfoT "Do not save the Markov chain."
 
 -- | Execute the 'Monitor's of the chain. See 'mExec'.
 mcmcMonitorExec :: ToJSON a => Mcmc a ()
diff --git a/src/Mcmc/Metropolis.hs b/src/Mcmc/Metropolis.hs
--- a/src/Mcmc/Metropolis.hs
+++ b/src/Mcmc/Metropolis.hs
@@ -91,12 +91,14 @@
   | t <= 0 = error "mhBurnInN: Auto tuning period smaller equal 0."
   | b > t = do
     mcmcResetA
+    mcmcMonitorStdOutHeader
     mhNIter t
     mcmcSummarizeCycle >>= mcmcDebugT
     mcmcAutotune
     mhBurnInN (b - t) (Just t)
   | otherwise = do
     mcmcResetA
+    mcmcMonitorStdOutHeader
     mhNIter b
     mcmcSummarizeCycle >>= mcmcInfoT
     mcmcInfoS $ "Acceptance ratios calculated over the last " <> show b <> " iterations."
@@ -110,16 +112,22 @@
   | otherwise = do
     mcmcInfoS $ "Burn in for " <> show b <> " cycles."
     mcmcDebugS $ "Auto tuning period is " <> show t <> "."
-    mcmcMonitorStdOutHeader
     mhBurnInN b t
     mcmcInfoT "Burn in finished."
 
 -- Run for given number of iterations.
 mhRun :: ToJSON a => Int -> Mcmc a ()
 mhRun n = do
+  mcmcResetA
   mcmcInfoS $ "Run chain for " <> show n <> " iterations."
-  mcmcMonitorStdOutHeader
-  mhNIter n
+  let (m, r) = n `quotRem` 100
+  -- Print header to standard output every 100 iterations.
+  replicateM_ m $ do
+    mcmcMonitorStdOutHeader
+    mhNIter 100
+  when (r > 0) $ do
+    mcmcMonitorStdOutHeader
+    mhNIter r
 
 mhT :: ToJSON a => Mcmc a ()
 mhT = do
@@ -149,8 +157,9 @@
 mhContinue dn s
   | dn <= 0 = error "mhContinue: The number of iterations is zero or negative."
   | otherwise = mcmcRun (mhContinueT dn) s'
-    where n' = iterations s + dn
-          s' = s {iterations = n'}
+  where
+    n' = iterations s + dn
+    s' = s {iterations = n'}
 
 -- | Run a Markov chain for a given number of Metropolis-Hastings steps.
 mh ::
diff --git a/src/Mcmc/Monitor.hs b/src/Mcmc/Monitor.hs
--- a/src/Mcmc/Monitor.hs
+++ b/src/Mcmc/Monitor.hs
@@ -30,6 +30,7 @@
   )
 where
 
+import Control.Concurrent.Async
 import Control.Monad
 import qualified Data.ByteString.Builder as BB
 import qualified Data.ByteString.Lazy.Char8 as BL
@@ -310,7 +311,7 @@
           renderLog mlos :
             [BB.toLazyByteString $ mbpFunc mbp (map state t) | mbp <- mbParams m]
   where
-    t = takeT (mbSize m) t'
+    t = takeItems (mbSize m) t'
     lps = map prior t
     lls = map likelihood t
     los = zipWith (*) lps lls
@@ -362,11 +363,15 @@
   Monitor a ->
   IO (Maybe BL.ByteString)
 mExec v i ss st xs j (Monitor s fs bs) = do
-  mapM_ (mfExec i $ headT xs) fs
-  mapM_ (mbExec i xs) bs
-  if v == Quiet
-    then return Nothing
-    else msExec i (headT xs) ss st j s
+  -- XXX: I am not sure if this concurrency is necessary.
+  mf <- async $ mapConcurrently_ (mfExec i $ headT xs) fs
+  mb <- async $ mapConcurrently_ (mbExec i xs) bs
+  ms <- async $ if v == Quiet
+                then return Nothing
+                else msExec i (headT xs) ss st j s
+  wait mf
+  wait mb
+  wait ms
 
 -- | Close the files associated with the 'Monitor'.
 mClose :: Monitor a -> IO (Monitor a)
diff --git a/src/Mcmc/Proposal.hs b/src/Mcmc/Proposal.hs
--- a/src/Mcmc/Proposal.hs
+++ b/src/Mcmc/Proposal.hs
@@ -168,9 +168,15 @@
     return $ m {pSimple = f t', pTuner = Just $ Tuner t' f}
 
 -- XXX: The desired acceptance ratio 0.44 is optimal for one-dimensional
--- 'Proposal's; one could also store the affected number of dimensions with the
--- 'Proposal' and tune towards an acceptance ratio accounting for the number of
+-- proposals; one could also store the affected number of dimensions with the
+-- proposal and tune towards an acceptance ratio accounting for the number of
 -- dimensions.
+--
+-- The optimal ratios seem to be:
+-- - One dimension: 0.44 (numerical result).
+-- - Five and more dimensions: 0.234 seems to be a good value (numerical result).
+-- - Infinite dimensions: 0.234 (theorem for specific target distributions).
+-- See Handbook of Markov chain Monte Carlo, chapter 4.
 ratioOpt :: Double
 ratioOpt = 0.44
 
diff --git a/src/Mcmc/Save.hs b/src/Mcmc/Save.hs
--- a/src/Mcmc/Save.hs
+++ b/src/Mcmc/Save.hs
@@ -26,6 +26,7 @@
 import Control.Monad
 import Data.Aeson
 import Data.Aeson.TH
+import Data.Maybe
 import qualified Data.ByteString.Lazy.Char8 as BL
 import Data.List hiding (cycle)
 import qualified Data.Map as M
@@ -40,6 +41,7 @@
 import Mcmc.Trace
 import Mcmc.Verbosity
 import Numeric.Log
+import System.Directory
 import System.IO.Unsafe (unsafePerformIO)
 import System.Random.MWC
 import Prelude hiding (cycle)
@@ -56,7 +58,7 @@
       (Maybe Int) -- Auto tune.
       Int -- Iterations.
       Bool -- Force.
-      Bool -- Save.
+      (Maybe Int) -- Save.
       Verbosity
       (Vector Word32) -- Current seed.
 
@@ -71,7 +73,7 @@
     nm
     it
     i
-    tr
+    tr'
     ac'
     br
     at
@@ -82,6 +84,7 @@
     g'
     ts
   where
+    tr' = takeT (fromMaybe 0 sv) tr
     ac' = transformKeysA (ccProposals c) [0 ..] ac
     -- TODO: Splitmix. Remove as soon as split mix is used and is available with
     -- the statistics package.
@@ -142,12 +145,15 @@
     c' = tuneCycle (M.mapMaybe id $ M.fromList $ zip (ccProposals c) ts) c
 
 -- | Load a 'Status' from file.
+--
 -- Important information that cannot be saved and has to be provided again when
 -- a chain is restored:
 -- - prior function
 -- - likelihood function
 -- - cycle
 -- - monitor
+--
+-- To avoid incomplete continued runs, the 'mcmc' file is removed after load.
 loadStatus ::
   FromJSON a =>
   (a -> Log Double) ->
@@ -177,4 +183,5 @@
     ( error
         "loadStatus: Provided likelihood function does not match the saved likelihood."
     )
+  removeFile fn
   return s
diff --git a/src/Mcmc/Status.hs b/src/Mcmc/Status.hs
--- a/src/Mcmc/Status.hs
+++ b/src/Mcmc/Status.hs
@@ -28,7 +28,7 @@
 module Mcmc.Status
   ( Status (..),
     status,
-    noSave,
+    saveWith,
     force,
     quiet,
     debug,
@@ -49,64 +49,68 @@
 
 -- | The 'Status' contains all information to run an MCMC chain. It is
 -- constructed using the function 'status'.
-data Status a
-  = Status
-      { -- MCMC related variables; saved.
+data Status a = Status
+  { -- MCMC related variables; saved.
 
-        -- | The name of the MCMC chain; used as file prefix.
-        name :: String,
-        -- | The current 'Item' of the chain combines the current state and the
-        -- current likelihood.
-        item :: Item a,
-        -- | The iteration is the number of completed cycles.
-        iteration :: Int,
-        -- | The 'Trace' of the Markov chain in reverse order, the most recent
-        -- 'Item' is at the head of the list.
-        trace :: Trace a,
-        -- | For each 'Proposal', store the list of accepted (True) and rejected (False)
-        -- proposals; for reasons of efficiency, the list is also stored in reverse
-        -- order.
-        acceptance :: Acceptance (Proposal a),
-        -- | Number of burn in iterations; deactivate burn in with 'Nothing'.
-        burnInIterations :: Maybe Int,
-        -- | Auto tuning period (only during burn in); deactivate auto tuning with
-        -- 'Nothing'.
-        autoTuningPeriod :: Maybe Int,
-        -- | Number of normal iterations excluding burn in. Note that auto tuning
-        -- only happens during burn in.
-        iterations :: Int,
-        -- Auxiliary variables; saved.
+    -- | The name of the MCMC chain; used as file prefix.
+    name :: String,
+    -- | The current 'Item' of the chain combines the current state and the
+    -- current likelihood.
+    item :: Item a,
+    -- | The iteration is the number of completed cycles.
+    iteration :: Int,
+    -- | The 'Trace' of the Markov chain in reverse order, the most recent
+    -- 'Item' is at the head of the list.
+    trace :: Trace a,
+    -- | For each 'Proposal', store the list of accepted (True) and rejected (False)
+    -- proposals; for reasons of efficiency, the list is also stored in reverse
+    -- order.
+    acceptance :: Acceptance (Proposal a),
+    -- | Number of burn in iterations; deactivate burn in with 'Nothing'.
+    burnInIterations :: Maybe Int,
+    -- | Auto tuning period (only during burn in); deactivate auto tuning with
+    -- 'Nothing'.
+    autoTuningPeriod :: Maybe Int,
+    -- | Number of normal iterations excluding burn in. Note that auto tuning
+    -- only happens during burn in.
+    iterations :: Int,
+    --
+    -- Auxiliary variables; saved.
 
-        -- | Overwrite output files? Default is 'False', change with 'force'.
-        forceOverwrite :: Bool,
-        -- | Save the chain at the end of the run? Default is 'True', change with 'noSave'.
-        save :: Bool,
-        -- | Verbosity.
-        verbosity :: Verbosity,
-        -- | The random number generator.
-        generator :: GenIO,
-        -- Auxiliary variables; not saved.
+    -- | Overwrite output files? Default is 'False', change with 'force'.
+    forceOverwrite :: Bool,
+    -- | Save the chain with trace of given length at the end of the run?
+    -- Default is no save ('Nothing'). Change with 'saveWith'.
+    save :: Maybe Int,
+    -- | Verbosity.
+    verbosity :: Verbosity,
+    -- | The random number generator.
+    generator :: GenIO,
+    --
+    -- Auxiliary variables; not saved.
 
-        -- | Starting time and starting iteration of chain; used to calculate
-        -- run time and ETA.
-        start :: Maybe (Int, UTCTime),
-        -- | Handle to log file.
-        logHandle :: Maybe Handle,
-        -- Auxiliary functions; not saved.
+    -- | Starting time and starting iteration of chain; used to calculate
+    -- run time and ETA.
+    start :: Maybe (Int, UTCTime),
+    -- | Handle to log file.
+    logHandle :: Maybe Handle,
+    --
+    -- Auxiliary functions; not saved.
 
-        -- | The prior function. The un-normalized posterior is the product of the
-        -- prior and the likelihood.
-        priorF :: a -> Log Double,
-        -- | The likelihood function. The un-normalized posterior is the product of
-        -- the prior and the likelihood.
-        likelihoodF :: a -> Log Double,
-        -- Variables related to the algorithm; not saved.
+    -- | The prior function. The un-normalized posterior is the product of the
+    -- prior and the likelihood.
+    priorF :: a -> Log Double,
+    -- | The likelihood function. The un-normalized posterior is the product of
+    -- the prior and the likelihood.
+    likelihoodF :: a -> Log Double,
+    --
+    -- Variables related to the algorithm; not saved.
 
-        -- | A set of 'Proposal's form a 'Cycle'.
-        cycle :: Cycle a,
-        -- | A 'Monitor' observing the chain.
-        monitor :: Monitor a
-      }
+    -- | A set of 'Proposal's form a 'Cycle'.
+    cycle :: Cycle a,
+    -- | A 'Monitor' observing the chain.
+    monitor :: Monitor a
+  }
 
 -- | Initialize the 'Status' of a Markov chain Monte Carlo run.
 status ::
@@ -148,7 +152,7 @@
       mT
       nI
       False
-      True
+      Nothing
       Info
       g
       Nothing
@@ -160,9 +164,9 @@
   where
     i = Item x (p x) (l x)
 
--- | Do not save the Markov chain at the end.
-noSave :: Status a -> Status a
-noSave s = s {save = False}
+-- | Save the Markov chain with trace of given length.
+saveWith :: Int -> Status a -> Status a
+saveWith n s = s {save = Just n}
 
 -- | Overwrite existing files; it is not necessary to use 'force', when a chain
 -- is continued.
diff --git a/src/Mcmc/Trace.hs b/src/Mcmc/Trace.hs
--- a/src/Mcmc/Trace.hs
+++ b/src/Mcmc/Trace.hs
@@ -14,6 +14,7 @@
     singletonT,
     pushT,
     headT,
+    takeItems,
     takeT,
   )
 where
@@ -54,5 +55,9 @@
 headT = head . fromTrace
 
 -- | Get the N most recent items of the trace.
-takeT :: Int -> Trace a -> [Item a]
-takeT n = take n . fromTrace
+takeItems :: Int -> Trace a -> [Item a]
+takeItems n = take n . fromTrace
+
+-- | Shorten the trace to given length.
+takeT :: Int -> Trace a -> Trace a
+takeT n = Trace . take n . fromTrace
diff --git a/test/Mcmc/SaveSpec.hs b/test/Mcmc/SaveSpec.hs
--- a/test/Mcmc/SaveSpec.hs
+++ b/test/Mcmc/SaveSpec.hs
@@ -23,7 +23,6 @@
     stdDev,
   )
 import Statistics.Distribution.Normal
-import System.Directory
 import System.Random.MWC
 import Test.Hspec
 
@@ -67,13 +66,14 @@
     $ do
       gen <- create
       let s =
-            force $ quiet $ noSave $
+            force $ quiet $ saveWith 100 $
               status "SaveSpec" (const 1) lh proposals mon 0 nBurn nAutoTune nIter gen
       saveStatus "SaveSpec.json" s
       s' <- loadStatus (const 1) lh proposals mon "SaveSpec.json"
       r <- mh s
       r' <- mh s'
-      removeFile "SaveSpec.json"
+      -- Done during 'loadStatus'.
+      -- removeFile "SaveSpec.json"
       item r `shouldBe` item r'
       iteration r `shouldBe` iteration r'
       trace r `shouldBe` trace r'
@@ -86,10 +86,10 @@
 --   $ it "mh 200 + mhContinue 200 == mh 400"
 --   $ do
 --     gen1 <- create
---     let s1 = noSave $ status "SaveSpec" (const 1) likelihood proposals mon 0 nBurn nAutoTune 400 gen1
+--     let s1 = status "SaveSpec" (const 1) likelihood proposals mon 0 nBurn nAutoTune 400 gen1
 --     r1 <- mh s1
 --     gen2 <- create
---     let s2 = noSave $ status "SaveSpec" (const 1) likelihood proposals mon 0 nBurn nAutoTune 200 gen2
+--     let s2 = status "SaveSpec" (const 1) likelihood proposals mon 0 nBurn nAutoTune 200 gen2
 --     r2' <- mh s2
 --     r2 <- mhContinue 200 r2'
 --     item r1 `shouldBe` item r2