diff --git a/doc/manual/conf-file.xml b/doc/manual/conf-file.xml
index 629dd3bac..9ff34324f 100644
--- a/doc/manual/conf-file.xml
+++ b/doc/manual/conf-file.xml
@@ -52,6 +52,26 @@ env-keep-derivations = false
 
   </varlistentry>
 
+  <varlistentry id="conf-gc-reserved-space"><term><literal>gc-reserved-space</literal></term>
+
+    <listitem><para>This option specifies how much space should be
+    reserved in normal use so that the garbage collector can run
+    succesfully.  Since the garbage collector must perform Berkeley DB
+    transactions, it needs some disk space for itself.  However, when
+    the disk is full, this space is not available, so the collector
+    would not be able to run precisely when it is most needed.</para>
+
+    <para>For this reason, when Nix is run, it allocates a file
+    <filename>/nix/var/nix/db/reserved</filename> of the size
+    specified by this option.  When the garbage collector is run, this
+    file is deleted before the Berkeley DB environment is opened.
+    This should give it enough room to proceed.</para>
+
+    <para>The default is <literal>1048576</literal> (1
+    MiB).</para></listitem>
+
+  </varlistentry>
+
   <varlistentry><term><literal>env-keep-derivations</literal></term>
 
     <listitem><para>If <literal>false</literal> (default), derivations
diff --git a/nix.conf.example b/nix.conf.example
index e2735d180..97c6f4b15 100644
--- a/nix.conf.example
+++ b/nix.conf.example
@@ -11,7 +11,7 @@
 # build time (e.g., the C compiler, or source tarballs downloaded from
 # the network).  To prevent it from doing so, set this option to
 # `true'.
-gc-keep-outputs = false
+#gc-keep-outputs = false
 
 
 ### Option `gc-keep-derivations'
@@ -26,7 +26,26 @@ gc-keep-outputs = false
 # store path was built), so by default this option is on.  Turn it off
 # to safe a bit of disk space (or a lot if `gc-keep-outputs' is also
 # turned on).
-gc-keep-derivations = true
+#gc-keep-derivations = true
+
+
+### Option `gc-reserved-space'
+#
+# This option specifies how much space should be reserved in normal
+# use so that the garbage collector can run succesfully.  Since the
+# garbage collector must perform Berkeley DB transactions, it needs
+# some disk space for itself.  However, when the disk is full, this
+# space is not available, so the collector would not be able to run
+# precisely when it is most needed.
+#
+# For this reason, when Nix is run, it allocates a file
+# /nix/var/nix/db/reserved of the size specified by this option.  When
+# the garbage collector is run, this file is deleted before the
+# Berkeley DB environment is opened.  This should give it enough room
+# to proceed.
+#
+# The default is "1048576" (1 MiB).
+#gc-reserved-space = 1048576
 
 
 ### Option `env-keep-derivations'
@@ -46,7 +65,7 @@ gc-keep-derivations = true
 # this one is `sticky': it applies to any user environment created
 # while this option was enabled, while `gc-keep-derivations' only
 # applies at the moment the garbage collector is run.
-env-keep-derivations = false
+#env-keep-derivations = false
 
 
 ### Option `build-allow-root'
@@ -56,7 +75,7 @@ env-keep-derivations = false
 # performed under the `root' user.  If `false', builds are performed
 # under one of the users listed in the `build-users' option (see
 # below).
-build-allow-root = true
+#build-allow-root = true
 
 
 ### Option `build-users'
@@ -77,4 +96,4 @@ build-allow-root = true
 #
 # Example:
 #   build-users = nix-builder-1 nix-builder-2 nix-builder-3
-build-users =
+#build-users =
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index a69bc0c30..fc338892f 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -75,17 +75,22 @@ Strings querySetting(const string & name, const Strings & def)
 }
 
 
-bool queryBoolSetting(const string & name, bool def)
+string querySetting(const string & name, const string & def)
 {
     Strings defs;
-    if (def) defs.push_back("true"); else defs.push_back("false");
-    
+    defs.push_back(def);
+
     Strings value = querySetting(name, defs);
     if (value.size() != 1)
-        throw Error(format("configuration option `%1%' should be either `true' or `false', not a list")
-            % name);
-    
-    string v = value.front();
+        throw Error(format("configuration option `%1%' should not be a list") % name);
+
+    return value.front();
+}
+
+
+bool queryBoolSetting(const string & name, bool def)
+{
+    string v = querySetting(name, def ? "true" : "false");
     if (v == "true") return true;
     else if (v == "false") return false;
     else throw Error(format("configuration option `%1%' should be either `true' or `false', not `%2%'")
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index cb199fd36..b5de709f7 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -56,6 +56,8 @@ extern bool readOnlyMode;
 
 Strings querySetting(const string & name, const Strings & def);
 
+string querySetting(const string & name, const string & def);
+
 bool queryBoolSetting(const string & name, bool def);
 
 
diff --git a/src/libstore/store.cc b/src/libstore/store.cc
index 25e2d6e36..94c992f22 100644
--- a/src/libstore/store.cc
+++ b/src/libstore/store.cc
@@ -82,12 +82,14 @@ void openDB(bool reserveSpace)
 
     try {
         Path reservedPath = nixDBPath + "/reserved";
-        off_t reservedSize = 1024 * 1024;
+        string s = querySetting("gc-reserved-space", "");
+        int reservedSize;
+        if (!string2Int(s, reservedSize)) reservedSize = 1024 * 1024;
         if (reserveSpace) {
             struct stat st;
             if (stat(reservedPath.c_str(), &st) == -1 ||
                 st.st_size != reservedSize)
-                writeFile(reservedPath, string(1024 * 1024, 'X'));
+                writeFile(reservedPath, string(reservedSize, 'X'));
         }
         else
             deletePath(reservedPath);