diff --git a/scripts/nix-build.in b/scripts/nix-build.in
index aa3f4661ae..c46b270ff8 100755
--- a/scripts/nix-build.in
+++ b/scripts/nix-build.in
@@ -46,7 +46,7 @@ Flags:
   --no-out-link: do not create the `result' symlink
   --out-link / -o NAME: create symlink NAME instead of `result'
   --attr / -A ATTR: select a specific attribute from the Nix expression
-  
+
   --run-env: build dependencies of the specified derivation, then start a
       shell with the environment of the derivation
   --command: command to run with `--run-env'
@@ -119,7 +119,7 @@ EOF
         push @buildArgs, "--dry-run";
         $dryRun = 1;
     }
-    
+
     elsif ($arg eq "--show-trace") {
         push @instArgs, $arg;
     }
@@ -127,22 +127,22 @@ EOF
     elsif ($arg eq "-") {
         @exprs = ("-");
     }
-    
+
     elsif ($arg eq "--verbose" or substr($arg, 0, 2) eq "-v") {
         push @buildArgs, $arg;
         push @instArgs, $arg;
         $verbose = 1;
     }
-    
+
     elsif ($arg eq "--quiet") {
         push @buildArgs, $arg;
         push @instArgs, $arg;
     }
-    
+
     elsif ($arg eq "--run-env") {
         $runEnv = 1;
     }
-    
+
     elsif ($arg eq "--command") {
         $n++;
         die "$0: `$arg' requires an argument\n" unless $n < scalar @ARGV;
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 74f7560fe0..c10177223e 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -144,6 +144,8 @@ EvalState::EvalState()
 {
     nrEnvs = nrValuesInEnvs = nrValues = nrListElems = 0;
     nrAttrsets = nrOpUpdates = nrOpUpdateValuesCopied = 0;
+    nrListConcats = nrPrimOpCalls = nrFunctionCalls = 0;
+    countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
 
 #if HAVE_BOEHMGC
     static bool gcInitialised = true;
@@ -300,8 +302,10 @@ inline Value * EvalState::lookupVar(Env * env, const VarRef & var)
     if (var.fromWith) {
         while (1) {
             Bindings::iterator j = env->values[0]->attrs->find(var.name);
-            if (j != env->values[0]->attrs->end())
+            if (j != env->values[0]->attrs->end()) {
+                if (countCalls && j->pos) attrSelects[*j->pos]++;
                 return j->value;
+            }
             if (env->prevWith == 0)
                 throwEvalError("undefined variable `%1%'", var.name);
             for (unsigned int l = env->prevWith; l; --l, env = env->up) ;
@@ -344,7 +348,7 @@ void EvalState::mkList(Value & v, unsigned int length)
 {
     v.type = tList;
     v.list.length = length;
-    v.list.elems = (Value * *) GC_MALLOC(length * sizeof(Value *));
+    v.list.elems = length ? (Value * *) GC_MALLOC(length * sizeof(Value *)) : 0;
     nrListElems += length;
 }
 
@@ -619,8 +623,10 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
             }
             vAttrs = j->value;
             pos = j->pos;
+            if (state.countCalls && pos) state.attrSelects[*pos]++;
         }
     
+
         state.forceValue(*vAttrs);
         
     } catch (Error & e) {
@@ -700,6 +706,8 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
                 vArgs[n--] = arg->primOpApp.right;
 
             /* And call the primop. */
+            nrPrimOpCalls++;
+            if (countCalls) primOpCalls[primOp->primOp->name]++;
             try {
                 primOp->primOp->fun(*this, vArgs, v);
             } catch (Error & e) {
@@ -716,7 +724,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
     }
     
     if (fun.type != tLambda)
-        throwTypeError("attempt to call something which is neither a function nor a primop (built-in operation) but %1%",
+        throwTypeError("attempt to call something which is not a function but %1%",
             showType(fun));
 
     unsigned int size =
@@ -760,6 +768,9 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
             throwTypeError("function at %1% called with unexpected argument", fun.lambda.fun->pos);
     }
 
+    nrFunctionCalls++;
+    if (countCalls) functionCalls[fun.lambda.fun->pos]++;
+
     try {
         fun.lambda.fun->body->eval(*this, env2, v);
     } catch (Error & e) {
@@ -902,14 +913,36 @@ void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1; e1->eval(state, env, v1);
-    state.forceList(v1);
     Value v2; e2->eval(state, env, v2);
-    state.forceList(v2);
-    state.mkList(v, v1.list.length + v2.list.length);
-    for (unsigned int n = 0; n < v1.list.length; ++n)
-        v.list.elems[n] = v1.list.elems[n];
-    for (unsigned int n = 0; n < v2.list.length; ++n)
-        v.list.elems[n + v1.list.length] = v2.list.elems[n];
+    Value * lists[2] = { &v1, &v2 };
+    state.concatLists(v, 2, lists);
+}
+
+
+void EvalState::concatLists(Value & v, unsigned int nrLists, Value * * lists)
+{
+    nrListConcats++;
+
+    Value * nonEmpty = 0;
+    unsigned int len = 0;
+    for (unsigned int n = 0; n < nrLists; ++n) {
+        forceList(*lists[n]);
+        unsigned int l = lists[n]->list.length;
+        len += l;
+        if (l) nonEmpty = lists[n];
+    }
+
+    if (nonEmpty && len == nonEmpty->list.length) {
+        v = *nonEmpty;
+        return;
+    }
+
+    mkList(v, len);
+    for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
+        unsigned int l = lists[n]->list.length;
+        memcpy(v.list.elems + pos, lists[n]->list.elems, l * sizeof(Value *));
+        pos += l;
+    }
 }
 
 
@@ -932,7 +965,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
             isPath = vStr.type == tPath;
             first = false;
         }
-            
+
         s << state.coerceToString(vStr, context, false, !isPath);
     }
         
@@ -1207,6 +1240,7 @@ void EvalState::printStats()
         % nrEnvs % (nrEnvs * sizeof(Env) + nrValuesInEnvs * sizeof(Value *)));
     printMsg(v, format("  list elements: %1% (%2% bytes)")
         % nrListElems % (nrListElems * sizeof(Value *)));
+    printMsg(v, format("  list concatenations: %1%") % nrListConcats);
     printMsg(v, format("  values allocated: %1% (%2% bytes)")
         % nrValues % (nrValues * sizeof(Value)));
     printMsg(v, format("  attribute sets allocated: %1%") % nrAttrsets);
@@ -1216,6 +1250,36 @@ void EvalState::printStats()
     printMsg(v, format("  number of thunks: %1%") % nrThunks);
     printMsg(v, format("  number of thunks avoided: %1%") % nrAvoided);
     printMsg(v, format("  number of attr lookups: %1%") % nrLookups);
+    printMsg(v, format("  number of primop calls: %1%") % nrPrimOpCalls);
+    printMsg(v, format("  number of function calls: %1%") % nrFunctionCalls);
+
+    if (countCalls) {
+
+        printMsg(v, format("calls to %1% primops:") % primOpCalls.size());
+        typedef std::multimap<unsigned int, Symbol> PrimOpCalls_;
+        std::multimap<unsigned int, Symbol> primOpCalls_;
+        foreach (PrimOpCalls::iterator, i, primOpCalls)
+            primOpCalls_.insert(std::pair<unsigned int, Symbol>(i->second, i->first));
+        foreach_reverse (PrimOpCalls_::reverse_iterator, i, primOpCalls_)
+            printMsg(v, format("%1$10d %2%") % i->first % i->second);
+
+        printMsg(v, format("calls to %1% functions:") % functionCalls.size());
+        typedef std::multimap<unsigned int, Pos> FunctionCalls_;
+        std::multimap<unsigned int, Pos> functionCalls_;
+        foreach (FunctionCalls::iterator, i, functionCalls)
+            functionCalls_.insert(std::pair<unsigned int, Pos>(i->second, i->first));
+        foreach_reverse (FunctionCalls_::reverse_iterator, i, functionCalls_)
+            printMsg(v, format("%1$10d %2%") % i->first % i->second);
+
+        printMsg(v, format("evaluations of %1% attributes:") % attrSelects.size());
+        typedef std::multimap<unsigned int, Pos> AttrSelects_;
+        std::multimap<unsigned int, Pos> attrSelects_;
+        foreach (AttrSelects::iterator, i, attrSelects)
+            attrSelects_.insert(std::pair<unsigned int, Pos>(i->second, i->first));
+        foreach_reverse (AttrSelects_::reverse_iterator, i, attrSelects_)
+            printMsg(v, format("%1$10d %2%") % i->first % i->second);
+
+    }
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 5103ae8cef..a1f26a0566 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -232,11 +232,13 @@ public:
     void mkAttrs(Value & v, unsigned int expected);
     void mkThunk_(Value & v, Expr * expr);
 
+    void concatLists(Value & v, unsigned int nrLists, Value * * lists);
+
     /* Print statistics. */
     void printStats();
 
 private:
-    
+
     unsigned long nrEnvs;
     unsigned long nrValuesInEnvs;
     unsigned long nrValues;
@@ -244,9 +246,25 @@ private:
     unsigned long nrAttrsets;
     unsigned long nrOpUpdates;
     unsigned long nrOpUpdateValuesCopied;
-    
-    friend class RecursionCounter;
+    unsigned long nrListConcats;
+    unsigned long nrPrimOpCalls;
+    unsigned long nrFunctionCalls;
+
+    bool countCalls;
+
+    typedef std::map<Symbol, unsigned int> PrimOpCalls;
+    PrimOpCalls primOpCalls;
+
+    typedef std::map<Pos, unsigned int> FunctionCalls;
+    FunctionCalls functionCalls;
+
+    typedef std::map<Pos, unsigned int> AttrSelects;
+    AttrSelects attrSelects;
+
     friend class ExprOpUpdate;
+    friend class ExprOpConcatLists;
+    friend class ExprSelect;
+    friend void prim_getAttr(EvalState & state, Value * * args, Value & v);
 };
 
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 4c1a0bb2d5..bc6c3287c7 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -27,6 +27,15 @@ struct Pos
     Pos() : line(0), column(0) { };
     Pos(const string & file, unsigned int line, unsigned int column)
         : file(file), line(line), column(column) { };
+    bool operator < (const Pos & p2) const
+    {
+        int d = file.compare(p2.file);
+        if (d < 0) return true;
+        if (d > 0) return false;
+        if (line < p2.line) return true;
+        if (line > p2.line) return false;
+        return column < p2.column;
+    }
 };
 
 extern Pos noPos;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 095e288430..1819da5e1c 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -203,7 +203,7 @@ static Expr * stripIndentation(SymbolTable & symbols, vector<Expr *> & es)
         es2->push_back(new ExprString(symbols.create(s2)));
     }
 
-    return new ExprConcatStrings(es2);
+    return es2->size() == 1 ? (*es2)[0] : new ExprConcatStrings(es2);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 7258c4cc0f..d3809e6984 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -719,7 +719,7 @@ static void prim_attrNames(EvalState & state, Value * * args, Value & v)
 
 
 /* Dynamic version of the `.' operator. */
-static void prim_getAttr(EvalState & state, Value * * args, Value & v)
+void prim_getAttr(EvalState & state, Value * * args, Value & v)
 {
     string attr = state.forceStringNoCtx(*args[0]);
     state.forceAttrs(*args[1]);
@@ -728,6 +728,7 @@ static void prim_getAttr(EvalState & state, Value * * args, Value & v)
     if (i == args[1]->attrs->end())
         throw EvalError(format("attribute `%1%' missing") % attr);
     // !!! add to stack trace?
+    if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
     state.forceValue(*i->value);
     v = *i->value;
 }
@@ -873,19 +874,33 @@ static void prim_isList(EvalState & state, Value * * args, Value & v)
 }
 
 
+static void elemAt(EvalState & state, Value & list, int n, Value & v)
+{
+    state.forceList(list);
+    if (n < 0 || n >= list.list.length)
+        throw Error(format("list index %1% is out of bounds") % n);
+    state.forceValue(*list.list.elems[n]);
+    v = *list.list.elems[n];
+}
+
+
+/* Return the n-1'th element of a list. */
+static void prim_elemAt(EvalState & state, Value * * args, Value & v)
+{
+    elemAt(state, *args[0], state.forceInt(*args[1]), v);
+}
+
+
 /* Return the first element of a list. */
 static void prim_head(EvalState & state, Value * * args, Value & v)
 {
-    state.forceList(*args[0]);
-    if (args[0]->list.length == 0)
-        throw Error("`head' called on an empty list");
-    state.forceValue(*args[0]->list.elems[0]);
-    v = *args[0]->list.elems[0];
+    elemAt(state, *args[0], 0, v);
 }
 
 
 /* Return a list consisting of everything but the the first element of
-   a list. */
+   a list.  Warning: this function takes O(n) time, so you probably
+   don't want to use it!  */
 static void prim_tail(EvalState & state, Value * * args, Value & v)
 {
     state.forceList(*args[0]);
@@ -911,6 +926,52 @@ static void prim_map(EvalState & state, Value * * args, Value & v)
 }
 
 
+/* Filter a list using a predicate; that is, return a list containing
+   every element from the list for which the predicate function
+   returns true. */
+static void prim_filter(EvalState & state, Value * * args, Value & v)
+{
+    state.forceFunction(*args[0]);
+    state.forceList(*args[1]);
+
+    // FIXME: putting this on the stack is risky.
+    Value * vs[args[1]->list.length];
+    unsigned int k = 0;
+
+    for (unsigned int n = 0; n < args[1]->list.length; ++n) {
+        Value res;
+        state.callFunction(*args[0], *args[1]->list.elems[n], res);
+        if (state.forceBool(res))
+            vs[k++] = args[1]->list.elems[n];
+    }
+
+    state.mkList(v, k);
+    for (unsigned int n = 0; n < k; ++n) v.list.elems[n] = vs[n];
+}
+
+
+/* Return true if a list contains a given element. */
+static void prim_elem(EvalState & state, Value * * args, Value & v)
+{
+    bool res = false;
+    state.forceList(*args[1]);
+    for (unsigned int n = 0; n < args[1]->list.length; ++n)
+        if (state.eqValues(*args[0], *args[1]->list.elems[n])) {
+            res = true;
+            break;
+        }
+    mkBool(v, res);
+}
+
+
+/* Concatenate a list of lists. */
+static void prim_concatLists(EvalState & state, Value * * args, Value & v)
+{
+    state.forceList(*args[0]);
+    state.concatLists(v, args[0]->list.length, args[0]->list.elems);
+}
+
+
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, Value * * args, Value & v)
 {
@@ -1122,11 +1183,15 @@ void EvalState::createBaseEnv()
 
     // Lists
     addPrimOp("__isList", 1, prim_isList);
+    addPrimOp("__elemAt", 2, prim_elemAt);
     addPrimOp("__head", 1, prim_head);
     addPrimOp("__tail", 1, prim_tail);
     addPrimOp("map", 2, prim_map);
+    addPrimOp("__filter", 2, prim_filter);
+    addPrimOp("__elem", 2, prim_elem);
+    addPrimOp("__concatLists", 1, prim_concatLists);
     addPrimOp("__length", 1, prim_length);
-    
+
     // Integer arithmetic
     addPrimOp("__add", 2, prim_add);
     addPrimOp("__sub", 2, prim_sub);
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index e1280911b3..5f92c3df34 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -33,16 +33,6 @@ static void sigintHandler(int signo)
 }
 
 
-Path makeRootName(const Path & gcRoot, int & counter)
-{
-    counter++;
-    if (counter == 1)
-        return gcRoot;
-    else
-        return (format("%1%-%2%") % gcRoot % counter).str();
-}
-
-
 void printGCWarning()
 {
     static bool haveWarned = false;
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index 7849e10e36..c69879a123 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -26,7 +26,6 @@ MakeError(UsageError, nix::Error);
 class StoreAPI;
 
 /* Ugh.  No better place to put this. */
-Path makeRootName(const Path & gcRoot, int & counter);
 void printGCWarning();
 
 void printMissing(StoreAPI & store, const PathSet & paths);
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 0972d6e193..1840fb7b21 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -45,7 +45,7 @@
 #include <sched.h>
 #endif
 
-#define CHROOT_ENABLED HAVE_CHROOT && HAVE_UNSHARE && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(CLONE_NEWNS)
+#define CHROOT_ENABLED HAVE_CHROOT && HAVE_UNSHARE && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(MS_PRIVATE) && defined(CLONE_NEWNS)
 
 #if CHROOT_ENABLED
 #include <sys/socket.h>
@@ -604,18 +604,17 @@ void getOwnership(const Path & path)
 }
 
 
-void deletePathWrapped(const Path & path,
-    unsigned long long & bytesFreed, unsigned long long & blocksFreed)
+void deletePathWrapped(const Path & path, unsigned long long & bytesFreed)
 {
     try {
         /* First try to delete it ourselves. */
-        deletePath(path, bytesFreed, blocksFreed);
+        deletePath(path, bytesFreed);
     } catch (SysError & e) {
         /* If this failed due to a permission error, then try it with
            the setuid helper. */
         if (settings.buildUsersGroup != "" && !amPrivileged()) {
             getOwnership(path);
-            deletePath(path, bytesFreed, blocksFreed);
+            deletePath(path, bytesFreed);
         } else
             throw;
     }
@@ -624,8 +623,8 @@ void deletePathWrapped(const Path & path,
 
 void deletePathWrapped(const Path & path)
 {
-    unsigned long long dummy1, dummy2;
-    deletePathWrapped(path, dummy1, dummy2);
+    unsigned long long dummy1;
+    deletePathWrapped(path, dummy1);
 }
 
 
@@ -1470,9 +1469,9 @@ HookReply DerivationGoal::tryBuildHook()
 }
 
 
-void chmod(const Path & path, mode_t mode)
+void chmod_(const Path & path, mode_t mode)
 {
-    if (::chmod(path.c_str(), 01777) == -1)
+    if (chmod(path.c_str(), mode) == -1)
         throw SysError(format("setting permissions on `%1%'") % path);
 }
 
@@ -1674,7 +1673,7 @@ void DerivationGoal::startBuilder()
            instead.) */
         Path chrootTmpDir = chrootRootDir + "/tmp";
         createDirs(chrootTmpDir);
-        chmod(chrootTmpDir, 01777);
+        chmod_(chrootTmpDir, 01777);
 
         /* Create a /etc/passwd with entries for the build user and the
            nobody account.  The latter is kind of a hack to support
@@ -1710,7 +1709,7 @@ void DerivationGoal::startBuilder()
            precaution, make the fake Nix store only writable by the
            build user. */
         createDirs(chrootRootDir + settings.nixStore);
-        chmod(chrootRootDir + settings.nixStore, 01777);
+        chmod_(chrootRootDir + settings.nixStore, 01777);
 
         foreach (PathSet::iterator, i, inputPaths) {
             struct stat st;
@@ -1844,22 +1843,40 @@ void DerivationGoal::initChild()
             char domainname[] = "(none)"; // kernel default
             setdomainname(domainname, sizeof(domainname));
 
+            /* Make all filesystems private.  This is necessary
+               because subtrees may have been mounted as "shared"
+               (MS_SHARED).  (Systemd does this, for instance.)  Even
+               though we have a private mount namespace, mounting
+               filesystems on top of a shared subtree still propagates
+               outside of the namespace.  Making a subtree private is
+               local to the namespace, though, so setting MS_PRIVATE
+               does not affect the outside world. */
+            Strings mounts = tokenizeString(readFile("/proc/self/mountinfo", true), "\n");
+            foreach (Strings::iterator, i, mounts) {
+                Strings fields = tokenizeString(*i, " ");
+                assert(fields.size() >= 5);
+                Strings::iterator j = fields.begin();
+                std::advance(j, 4);
+                if (mount(0, j->c_str(), 0, MS_PRIVATE, 0) == -1)
+                    throw SysError(format("unable to make filesystem `%1%' private") % *j);
+            }
+
             /* Bind-mount all the directories from the "host"
                filesystem that we want in the chroot
                environment. */
             foreach (PathSet::iterator, i, dirsInChroot) {
                 Path source = *i;
                 Path target = chrootRootDir + source;
+                if (source == "/proc") continue; // backwards compatibility
                 debug(format("bind mounting `%1%' to `%2%'") % source % target);
-
                 createDirs(target);
-
                 if (mount(source.c_str(), target.c_str(), "", MS_BIND, 0) == -1)
                     throw SysError(format("bind mount from `%1%' to `%2%' failed") % source % target);
             }
 
             /* Bind a new instance of procfs on /proc to reflect our
                private PID namespace. */
+            createDirs(chrootRootDir + "/proc");
             if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
                 throw SysError("mounting /proc");
 
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 1355702f87..88b7bec326 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -425,10 +425,9 @@ bool LocalStore::isActiveTempFile(const GCState & state,
 void LocalStore::deleteGarbage(GCState & state, const Path & path)
 {
     printMsg(lvlInfo, format("deleting `%1%'") % path);
-    unsigned long long bytesFreed, blocksFreed;
-    deletePathWrapped(path, bytesFreed, blocksFreed);
+    unsigned long long bytesFreed;
+    deletePathWrapped(path, bytesFreed);
     state.results.bytesFreed += bytesFreed;
-    state.results.blocksFreed += blocksFreed;
 }
 
 
@@ -550,7 +549,7 @@ bool LocalStore::tryToDelete(GCState & state, const Path & path)
         } else
             deleteGarbage(state, path);
 
-        if (state.options.maxFreed && state.results.bytesFreed + state.bytesInvalidated > state.options.maxFreed) {
+        if (state.results.bytesFreed + state.bytesInvalidated > state.options.maxFreed) {
             printMsg(lvlInfo, format("deleted or invalidated more than %1% bytes; stopping") % state.options.maxFreed);
             throw GCLimitReached();
         }
@@ -576,11 +575,13 @@ bool LocalStore::tryToDelete(GCState & state, const Path & path)
    safely deleted.  FIXME: race condition with optimisePath(): we
    might see a link count of 1 just before optimisePath() increases
    the link count. */
-void LocalStore::removeUnusedLinks()
+void LocalStore::removeUnusedLinks(const GCState & state)
 {
     AutoCloseDir dir = opendir(linksDir.c_str());
     if (!dir) throw SysError(format("opening directory `%1%'") % linksDir);
 
+    long long actualSize = 0, unsharedSize = 0;
+
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir)) {
         checkInterrupt();
@@ -592,13 +593,28 @@ void LocalStore::removeUnusedLinks()
         if (lstat(path.c_str(), &st) == -1)
             throw SysError(format("statting `%1%'") % path);
 
-        if (st.st_nlink != 1) continue;
+        if (st.st_nlink != 1) {
+            unsigned long long size = st.st_blocks * 512ULL;
+            actualSize += size;
+            unsharedSize += (st.st_nlink - 1) * size;
+            continue;
+        }
 
         printMsg(lvlTalkative, format("deleting unused link `%1%'") % path);
 
         if (unlink(path.c_str()) == -1)
             throw SysError(format("deleting `%1%'") % path);
+
+        state.results.bytesFreed += st.st_blocks * 512;
     }
+
+    struct stat st;
+    if (stat(linksDir.c_str(), &st) == -1)
+        throw SysError(format("statting `%1%'") % linksDir);
+    long long overhead = st.st_blocks * 512ULL;
+
+    printMsg(lvlInfo, format("note: currently hard linking saves %.2f MiB")
+        % ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
 }
 
 
@@ -660,7 +676,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                 throw Error(format("cannot delete path `%1%' since it is still alive") % *i);
         }
 
-    } else {
+    } else if (options.maxFreed > 0) {
 
         if (shouldDelete(state.options.action))
             printMsg(lvlError, format("deleting garbage..."));
@@ -718,7 +734,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 
     /* Clean up the links directory. */
     printMsg(lvlError, format("deleting unused links..."));
-    removeUnusedLinks();
+    removeUnusedLinks(state);
 }
 
 
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 3cb016e9ca..ba05829221 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -261,7 +261,7 @@ private:
 
     int openGCLock(LockType lockType);
 
-    void removeUnusedLinks();
+    void removeUnusedLinks(const GCState & state);
 
     void startSubstituter(const Path & substituter,
         RunningSubstituter & runningSubstituter);
@@ -298,8 +298,7 @@ void getOwnership(const Path & path);
 
 /* Like deletePath(), but changes the ownership of `path' using the
    setuid wrapper if necessary (and possible). */
-void deletePathWrapped(const Path & path,
-    unsigned long long & bytesFreed, unsigned long long & blocksFreed);
+void deletePathWrapped(const Path & path, unsigned long long & bytesFreed);
 
 void deletePathWrapped(const Path & path);
 
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index 334f4f355f..9d0242bbc8 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -102,11 +102,11 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
         /* Nope, create a hard link in the links directory. */
         makeMutable(path);
         MakeImmutable mk1(path);
-
-        if (link(path.c_str(), linkPath.c_str()) == -1)
+        if (link(path.c_str(), linkPath.c_str()) == 0) return;
+        if (errno != EEXIST)
             throw SysError(format("cannot link `%1%' to `%2%'") % linkPath % path);
-
-        return;
+        /* Fall through if another process created ‘linkPath’ before
+           we did. */
     }
 
     /* Yes!  We've seen a file with the same contents.  Replace the
@@ -123,9 +123,6 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
 
     printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % linkPath);
 
-    Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
-        % settings.nixStore % getpid() % rand()).str();
-
     /* Make the containing directory writable, but only if it's not
        the store itself (we don't want or need to mess with its
        permissions). */
@@ -140,40 +137,55 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
        so make it mutable first (and make it immutable again when
        we're done).  We also have to make ‘path’ mutable, otherwise
        rename() will fail to delete it. */
-    makeMutable(linkPath);
-    MakeImmutable mk1(linkPath);
-
     makeMutable(path);
     MakeImmutable mk2(path);
 
-    if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
-        if (errno == EMLINK) {
-            /* Too many links to the same file (>= 32000 on most file
-               systems).  This is likely to happen with empty files.
-               Just shrug and ignore. */
-            printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
-            return;
+    /* Another process might be doing the same thing (creating a new
+       link to ‘linkPath’) and make ‘linkPath’ immutable before we're
+       done.  In that case, just retry. */
+    unsigned int retries = 1024;
+    while (--retries > 0) {
+        makeMutable(linkPath);
+        MakeImmutable mk1(linkPath);
+
+        Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
+            % settings.nixStore % getpid() % rand()).str();
+
+        if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
+            if (errno == EMLINK) {
+                /* Too many links to the same file (>= 32000 on most
+                   file systems).  This is likely to happen with empty
+                   files.  Just shrug and ignore. */
+                if (st.st_size)
+                    printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
+                return;
+            }
+            if (errno == EPERM) continue;
+            throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
         }
-        throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
-    }
 
-    /* Atomically replace the old file with the new hard link. */
-    if (rename(tempLink.c_str(), path.c_str()) == -1) {
-        if (errno == EMLINK) {
-            /* Some filesystems generate too many links on the rename,
-               rather than on the original link.  (Probably it
-               temporarily increases the st_nlink field before
-               decreasing it again.) */
-            printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
-
-            /* Unlink the temp link. */
+        /* Atomically replace the old file with the new hard link. */
+        if (rename(tempLink.c_str(), path.c_str()) == -1) {
             if (unlink(tempLink.c_str()) == -1)
                 printMsg(lvlError, format("unable to unlink `%1%'") % tempLink);
-            return;
+            if (errno == EMLINK) {
+                /* Some filesystems generate too many links on the
+                   rename, rather than on the original link.
+                   (Probably it temporarily increases the st_nlink
+                   field before decreasing it again.) */
+                if (st.st_size)
+                    printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
+                return;
+            }
+            if (errno == EPERM) continue;
+            throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
         }
-        throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
+
+        break;
     }
 
+    if (retries == 0) throw Error(format("cannot link `%1%' to `%2%'") % path % linkPath);
+
     stats.filesLinked++;
     stats.bytesFreed += st.st_size;
     stats.blocksFreed += st.st_blocks;
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 56396541ad..d3c05f0df4 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -558,7 +558,7 @@ void RemoteStore::collectGarbage(const GCOptions & options, GCResults & results)
 
     results.paths = readStrings<PathSet>(from);
     results.bytesFreed = readLongLong(from);
-    results.blocksFreed = readLongLong(from);
+    readLongLong(from); // obsolete
 }
 
 
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 6f81a9aab0..32aaca6be0 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -2,7 +2,7 @@
 #include "globals.hh"
 #include "util.hh"
 
-#include <limits.h>
+#include <climits>
 
 
 namespace nix {
@@ -12,7 +12,7 @@ GCOptions::GCOptions()
 {
     action = gcDeleteDead;
     ignoreLiveness = false;
-    maxFreed = 0;
+    maxFreed = ULLONG_MAX;
 }
 
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 324d802dc4..a562360ce3 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -48,8 +48,7 @@ struct GCOptions
     /* For `gcDeleteSpecific', the paths to delete. */
     PathSet pathsToDelete;
 
-    /* Stop after at least `maxFreed' bytes have been freed.  0 means
-       no limit. */
+    /* Stop after at least `maxFreed' bytes have been freed. */
     unsigned long long maxFreed;
 
     GCOptions();
@@ -66,13 +65,9 @@ struct GCResults
        number of bytes that would be or was freed. */
     unsigned long long bytesFreed;
 
-    /* The number of file system blocks that would be or was freed. */
-    unsigned long long blocksFreed;
-
     GCResults()
     {
         bytesFreed = 0;
-        blocksFreed = 0;
     }
 };
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 3790e2fa3b..56bf5875de 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -224,12 +224,12 @@ string readFile(int fd)
 }
 
 
-string readFile(const Path & path)
+string readFile(const Path & path, bool drain)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY);
     if (fd == -1)
         throw SysError(format("opening file `%1%'") % path);
-    return readFile(fd);
+    return drain ? drainFD(fd) : readFile(fd);
 }
 
 
@@ -297,8 +297,7 @@ void computePathSize(const Path & path,
 }
 
 
-static void _deletePath(const Path & path, unsigned long long & bytesFreed,
-    unsigned long long & blocksFreed)
+static void _deletePath(const Path & path, unsigned long long & bytesFreed)
 {
     checkInterrupt();
 
@@ -308,10 +307,8 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed,
 
     if (S_ISDIR(st.st_mode) || S_ISREG(st.st_mode)) makeMutable(path);
 
-    if (!S_ISDIR(st.st_mode) && st.st_nlink == 1) {
-        bytesFreed += st.st_size;
-        blocksFreed += st.st_blocks;
-    }
+    if (!S_ISDIR(st.st_mode) && st.st_nlink == 1)
+        bytesFreed += st.st_blocks * 512;
 
     if (S_ISDIR(st.st_mode)) {
 	Strings names = readDirectory(path);
@@ -323,7 +320,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed,
 	}
 
 	for (Strings::iterator i = names.begin(); i != names.end(); ++i)
-            _deletePath(path + "/" + *i, bytesFreed, blocksFreed);
+            _deletePath(path + "/" + *i, bytesFreed);
     }
 
     if (remove(path.c_str()) == -1)
@@ -333,19 +330,17 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed,
 
 void deletePath(const Path & path)
 {
-    unsigned long long dummy1, dummy2;
-    deletePath(path, dummy1, dummy2);
+    unsigned long long dummy;
+    deletePath(path, dummy);
 }
 
 
-void deletePath(const Path & path, unsigned long long & bytesFreed,
-    unsigned long long & blocksFreed)
+void deletePath(const Path & path, unsigned long long & bytesFreed)
 {
     startNest(nest, lvlDebug,
         format("recursively deleting path `%1%'") % path);
     bytesFreed = 0;
-    blocksFreed = 0;
-    _deletePath(path, bytesFreed, blocksFreed);
+    _deletePath(path, bytesFreed);
 }
 
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 16633a0835..0616288cd5 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -17,6 +17,9 @@ namespace nix {
 #define foreach(it_type, it, collection)                                \
     for (it_type it = (collection).begin(); it != (collection).end(); ++it)
 
+#define foreach_reverse(it_type, it, collection)                                \
+    for (it_type it = (collection).rbegin(); it != (collection).rend(); ++it)
+
 
 /* Return an environment variable. */
 string getEnv(const string & key, const string & def = "");
@@ -60,7 +63,7 @@ Strings readDirectory(const Path & path);
 
 /* Read the contents of a file into a string. */
 string readFile(int fd);
-string readFile(const Path & path);
+string readFile(const Path & path, bool drain = false);
 
 /* Write a string to a file. */
 void writeFile(const Path & path, const string & s);
@@ -80,8 +83,7 @@ void computePathSize(const Path & path,
    returns the number of bytes and blocks freed. */
 void deletePath(const Path & path);
 
-void deletePath(const Path & path, unsigned long long & bytesFreed,
-    unsigned long long & blocksFreed);
+void deletePath(const Path & path, unsigned long long & bytesFreed);
 
 /* Make a path read-only recursively. */
 void makePathReadOnly(const Path & path);
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 270b4ddc2e..beb7fa0249 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -64,9 +64,11 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                     Path drvPath = i->queryDrvPath(state);
                     if (gcRoot == "")
                         printGCWarning();
-                    else
-                        drvPath = addPermRoot(*store, drvPath,
-                            makeRootName(gcRoot, rootNr), indirectRoot);
+                    else {
+                        Path rootName = gcRoot;
+                        if (++rootNr > 1) rootName += "-" + int2String(rootNr);
+                        drvPath = addPermRoot(*store, drvPath, rootName, indirectRoot);
+                    }
                     std::cout << format("%1%\n") % drvPath;
                 }
             }
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 5ada797136..d3a707f0d8 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -64,15 +64,19 @@ static PathSet realisePath(const Path & path)
     if (isDerivation(path)) {
         store->buildPaths(singleton<PathSet>(path));
         Derivation drv = derivationFromPath(*store, path);
+        rootNr++;
 
         PathSet outputs;
         foreach (DerivationOutputs::iterator, i, drv.outputs) {
             Path outPath = i->second.path;
             if (gcRoot == "")
                 printGCWarning();
-            else
-                outPath = addPermRoot(*store, outPath,
-                    makeRootName(gcRoot, rootNr), indirectRoot);
+            else {
+                Path rootName = gcRoot;
+                if (rootNr > 1) rootName += "-" + int2String(rootNr);
+                if (i->first != "out") rootName += "-" + i->first;
+                outPath = addPermRoot(*store, outPath, rootName, indirectRoot);
+            }
             outputs.insert(outPath);
         }
         return outputs;
@@ -544,10 +548,9 @@ static void opCheckValidity(Strings opFlags, Strings opArgs)
 }
 
 
-static string showBytes(unsigned long long bytes, unsigned long long blocks)
+static string showBytes(unsigned long long bytes)
 {
-    return (format("%d bytes (%.2f MiB, %d blocks)")
-        % bytes % (bytes / (1024.0 * 1024.0)) % blocks).str();
+    return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
 }
 
 
@@ -562,7 +565,7 @@ struct PrintFreed
         if (show)
             cout << format("%1% store paths deleted, %2% freed\n")
                 % results.paths.size()
-                % showBytes(results.bytesFreed, results.blocksFreed);
+                % showBytes(results.bytesFreed);
     }
 };
 
@@ -583,7 +586,7 @@ static void opGC(Strings opFlags, Strings opArgs)
         else if (*i == "--delete") options.action = GCOptions::gcDeleteDead;
         else if (*i == "--max-freed") {
             long long maxFreed = getIntArg<long long>(*i, i, opFlags.end());
-            options.maxFreed = maxFreed >= 1 ? maxFreed : 1;
+            options.maxFreed = maxFreed >= 0 ? maxFreed : 0;
         }
         else throw UsageError(format("bad sub-operation `%1%' in GC") % *i);
 
@@ -735,7 +738,7 @@ static void showOptimiseStats(OptimiseStats & stats)
 {
     printMsg(lvlError,
         format("%1% freed by hard-linking %2% files; there are %3% files with equal contents out of %4% files in total")
-        % showBytes(stats.bytesFreed, stats.blocksFreed)
+        % showBytes(stats.bytesFreed)
         % stats.filesLinked
         % stats.sameContents
         % stats.totalFiles);
diff --git a/src/nix-worker/nix-worker.cc b/src/nix-worker/nix-worker.cc
index 8ccafca29d..dadde9cc51 100644
--- a/src/nix-worker/nix-worker.cc
+++ b/src/nix-worker/nix-worker.cc
@@ -521,7 +521,7 @@ static void performOp(unsigned int clientVersion,
 
         writeStrings(results.paths, to);
         writeLongLong(results.bytesFreed, to);
-        writeLongLong(results.blocksFreed, to);
+        writeLongLong(0, to); // obsolete
 
         break;
     }
@@ -661,6 +661,10 @@ static void processConnection()
     to.flush();
     unsigned int clientVersion = readInt(from);
 
+    bool reserveSpace = true;
+    if (GET_PROTOCOL_MINOR(clientVersion) >= 11)
+        reserveSpace = readInt(from) != 0;
+
     /* Send startup error messages to the client. */
     startWork();
 
@@ -676,10 +680,6 @@ static void processConnection()
             throw Error("if you run `nix-worker' as root, then you MUST set `build-users-group'!");
 #endif
 
-        bool reserveSpace = true;
-        if (GET_PROTOCOL_MINOR(clientVersion) >= 11)
-            reserveSpace = readInt(from) != 0;
-
         /* Open the store. */
         store = boost::shared_ptr<StoreAPI>(new LocalStore(reserveSpace));