diff --git a/lib/spack/spack/__init__.py b/lib/spack/spack/__init__.py
index 50fe453cfb..9eae4342e3 100644
--- a/lib/spack/spack/__init__.py
+++ b/lib/spack/spack/__init__.py
@@ -81,7 +81,7 @@
 # stage directories.
 #
 from spack.directory_layout import SpecHashDirectoryLayout
-install_layout = SpecHashDirectoryLayout(install_path, prefix_size=6)
+install_layout = SpecHashDirectoryLayout(install_path)
 
 #
 # This controls how things are concretized in spack.
diff --git a/lib/spack/spack/cmd/dependents.py b/lib/spack/spack/cmd/dependents.py
index db6be88d32..129a4eeb23 100644
--- a/lib/spack/spack/cmd/dependents.py
+++ b/lib/spack/spack/cmd/dependents.py
@@ -29,7 +29,7 @@
 import spack
 import spack.cmd
 
-description = "Show dependent packages."
+description = "Show installed packages that depend on another."
 
 def setup_parser(subparser):
     subparser.add_argument(
@@ -42,5 +42,5 @@ def dependents(parser, args):
         tty.die("spack dependents takes only one spec.")
 
     fmt = '$_$@$%@$+$=$#'
-    deps = [d.format(fmt) for d in specs[0].package.installed_dependents]
-    tty.msg("Dependents of %s" % specs[0].format(fmt), *deps)
+    deps = [d.format(fmt, color=True) for d in specs[0].package.installed_dependents]
+    tty.msg("Dependents of %s" % specs[0].format(fmt, color=True), *deps)
diff --git a/lib/spack/spack/cmd/find.py b/lib/spack/spack/cmd/find.py
index 7f2bce119e..72df69d18a 100644
--- a/lib/spack/spack/cmd/find.py
+++ b/lib/spack/spack/cmd/find.py
@@ -89,7 +89,7 @@ def find(parser, args):
                 format = "    %-{}s%s".format(width)
 
                 for abbrv, spec in zip(abbreviated, specs):
-                    print format % (abbrv, spec.package.prefix)
+                    print format % (abbrv, spec.prefix)
 
             elif args.full_specs:
                 for spec in specs:
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index eb497711b7..e6d1bb87d4 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -118,7 +118,7 @@ def concretize_compiler(self, spec):
             return
 
         try:
-            nearest = next(p for p in spec.preorder_traversal(direction='parents')
+            nearest = next(p for p in spec.traverse(direction='parents')
                            if p.compiler is not None).compiler
 
             if not nearest in all_compilers:
diff --git a/lib/spack/spack/directory_layout.py b/lib/spack/spack/directory_layout.py
index 4fc00d536e..9b31aad5fe 100644
--- a/lib/spack/spack/directory_layout.py
+++ b/lib/spack/spack/directory_layout.py
@@ -29,7 +29,10 @@
 import shutil
 from contextlib import closing
 
+import llnl.util.tty as tty
 from llnl.util.filesystem import join_path, mkdirp
+
+import spack
 from spack.spec import Spec
 from spack.error import SpackError
 
@@ -131,12 +134,9 @@ def __init__(self, root, **kwargs):
         """Prefix size is number of characters in the SHA-1 prefix to use
            to make each hash unique.
         """
-        prefix_size = kwargs.get('prefix_size', 8)
-        spec_file   = kwargs.get('spec_file', '.spec')
-
+        spec_file_name   = kwargs.get('spec_file_name', '.spec')
         super(SpecHashDirectoryLayout, self).__init__(root)
-        self.prefix_size = prefix_size
-        self.spec_file = spec_file
+        self.spec_file_name = spec_file_name
 
 
     def relative_path_for_spec(self, spec):
@@ -154,16 +154,36 @@ def write_spec(self, spec, path):
     def read_spec(self, path):
         """Read the contents of a file and parse them as a spec"""
         with closing(open(path)) as spec_file:
-            string = spec_file.read().replace('\n', '')
             # Specs from files are assumed normal and concrete
-            return Spec(string, concrete=True)
+            spec = Spec(spec_file.read().replace('\n', ''))
+
+        # If we do not have a package on hand for this spec, we know
+        # it is concrete, and we *assume* that it is normal. This
+        # prevents us from trying to fetch a non-existing package, and
+        # allows best effort for commands like spack find.
+        if not spack.db.exists(spec.name):
+            spec._normal = True
+            spec._concrete = True
+        else:
+            spec.normalize()
+            if not spec.concrete:
+                tty.warn("Spec read from installed package is not concrete:",
+                         path, spec)
+
+        return spec
+
+
+    def spec_file_path(self, spec):
+        """Gets full path to spec file"""
+        _check_concrete(spec)
+        return join_path(self.path_for_spec(spec), self.spec_file_name)
 
 
     def make_path_for_spec(self, spec):
         _check_concrete(spec)
 
         path = self.path_for_spec(spec)
-        spec_file_path = join_path(path, self.spec_file)
+        spec_file_path = self.spec_file_path(spec)
 
         if os.path.isdir(path):
             if not os.path.isfile(spec_file_path):
@@ -177,8 +197,7 @@ def make_path_for_spec(self, spec):
             spec_hash = self.hash_spec(spec)
             installed_hash = self.hash_spec(installed_spec)
             if installed_spec == spec_hash:
-                raise SpecHashCollisionError(
-                    installed_hash, spec_hash, self.prefix_size)
+                raise SpecHashCollisionError(installed_hash, spec_hash)
             else:
                 raise InconsistentInstallDirectoryError(
                     'Spec file in %s does not match SHA-1 hash!'
@@ -195,7 +214,7 @@ def all_specs(self):
         for path in traverse_dirs_at_depth(self.root, 3):
             arch, compiler, last_dir = path
             spec_file_path = join_path(
-                self.root, arch, compiler, last_dir, self.spec_file)
+                self.root, arch, compiler, last_dir, self.spec_file_name)
             if os.path.exists(spec_file_path):
                 spec = self.read_spec(spec_file_path)
                 yield spec
@@ -209,10 +228,10 @@ def __init__(self, message):
 
 class SpecHashCollisionError(DirectoryLayoutError):
     """Raised when there is a hash collision in an SpecHashDirectoryLayout."""
-    def __init__(self, installed_spec, new_spec, prefix_size):
+    def __init__(self, installed_spec, new_spec):
         super(SpecHashDirectoryLayout, self).__init__(
-            'Specs %s and %s have the same %d character SHA-1 prefix!'
-            % prefix_size, installed_spec, new_spec)
+            'Specs %s and %s have the same SHA-1 prefix!'
+            % installed_spec, new_spec)
 
 
 class InconsistentInstallDirectoryError(DirectoryLayoutError):
diff --git a/lib/spack/spack/packages.py b/lib/spack/spack/packages.py
index 00834c95d5..ba997bf269 100644
--- a/lib/spack/spack/packages.py
+++ b/lib/spack/spack/packages.py
@@ -69,9 +69,9 @@ def get(self, spec):
 
         if not spec in self.instances:
             package_class = self.get_class_for_package_name(spec.name)
-            self.instances[spec.name] = package_class(spec)
+            self.instances[spec.copy()] = package_class(spec)
 
-        return self.instances[spec.name]
+        return self.instances[spec]
 
 
     @_autospec
@@ -115,7 +115,13 @@ def installed_package_specs(self):
         """Read installed package names straight from the install directory
            layout.
         """
-        return spack.install_layout.all_specs()
+        # Get specs from the directory layout but ensure that they're
+        # all normalized properly.
+        installed = []
+        for spec in spack.install_layout.all_specs():
+            spec.normalize()
+            installed.append(spec)
+        return installed
 
 
     @memoized
@@ -179,24 +185,6 @@ def get_class_for_package_name(self, pkg_name):
         return cls
 
 
-    def compute_dependents(self):
-        """Reads in all package files and sets dependence information on
-           Package objects in memory.
-        """
-        if not hasattr(compute_dependents, index):
-            compute_dependents.index = {}
-
-        for pkg in all_packages():
-            if pkg._dependents is None:
-                pkg._dependents = []
-
-            for name, dep in pkg.dependencies.iteritems():
-                dpkg = self.get(name)
-                if dpkg._dependents is None:
-                    dpkg._dependents = []
-                dpkg._dependents.append(pkg.name)
-
-
     def graph_dependencies(self, out=sys.stdout):
         """Print out a graph of all the dependencies between package.
            Graph is in dot format."""
@@ -211,10 +199,17 @@ def quote(string):
             return '"%s"' % string
 
         deps = []
-        for pkg in all_packages():
+        for pkg in self.all_packages():
             out.write('  %-30s [label="%s"]\n' % (quote(pkg.name), pkg.name))
+
+            # Add edges for each depends_on in the package.
             for dep_name, dep in pkg.dependencies.iteritems():
                 deps.append((pkg.name, dep_name))
+
+            # If the package provides something, add an edge for that.
+            for provider in set(p.name for p in pkg.provided):
+                deps.append((provider, pkg.name))
+
         out.write('\n')
 
         for pair in deps:
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 45c3402617..aa6397271b 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -94,6 +94,7 @@
 import itertools
 import hashlib
 from StringIO import StringIO
+from operator import attrgetter
 
 import llnl.util.tty as tty
 from llnl.util.lang import *
@@ -309,9 +310,8 @@ def concrete(self):
 
 
     def __str__(self):
-        sorted_dep_names = sorted(self.keys())
         return ''.join(
-            ["^" + str(self[name]) for name in sorted_dep_names])
+            ["^" + str(self[name]) for name in sorted(self.keys())])
 
 
 @key_ordering
@@ -352,10 +352,6 @@ def __init__(self, spec_like, *dep_like, **kwargs):
         self._normal = kwargs.get('normal', False)
         self._concrete = kwargs.get('concrete', False)
 
-        # Specs cannot be concrete and non-normal.
-        if self._concrete:
-            self._normal = True
-
         # This allows users to construct a spec DAG with literals.
         # Note that given two specs a and b, Spec(a) copies a, but
         # Spec(a, b) will copy a but just add b as a dep.
@@ -454,10 +450,19 @@ def concrete(self):
         return self._concrete
 
 
-    def preorder_traversal(self, visited=None, d=0, **kwargs):
-        """Generic preorder traversal of the DAG represented by this spec.
+    def traverse(self, visited=None, d=0, **kwargs):
+        """Generic traversal of the DAG represented by this spec.
            This will yield each node in the spec.  Options:
 
+           order    [=pre|post]
+               Order to traverse spec nodes. Defaults to preorder traversal.
+               Options are:
+
+               'pre':  Pre-order traversal; each node is yielded before its
+                       children in the dependency DAG.
+               'post': Post-order  traversal; each node is yielded after its
+                       children in the dependency DAG.
+
            cover    [=nodes|edges|paths]
                Determines how extensively to cover the dag.  Possible vlaues:
 
@@ -475,7 +480,7 @@ def preorder_traversal(self, visited=None, d=0, **kwargs):
                spec, but also their depth from the root in a (depth, node)
                tuple.
 
-           keyfun   [=id]
+           key   [=id]
                Allow a custom key function to track the identity of nodes
                in the traversal.
 
@@ -487,44 +492,57 @@ def preorder_traversal(self, visited=None, d=0, **kwargs):
                'parents', traverses upwards in the DAG towards the root.
 
         """
+        # get initial values for kwargs
         depth      = kwargs.get('depth', False)
         key_fun    = kwargs.get('key', id)
+        if isinstance(key_fun, basestring):
+            key_fun = attrgetter(key_fun)
         yield_root = kwargs.get('root', True)
         cover      = kwargs.get('cover', 'nodes')
         direction  = kwargs.get('direction', 'children')
+        order      = kwargs.get('order', 'pre')
 
-        cover_values = ('nodes', 'edges', 'paths')
-        if cover not in cover_values:
-            raise ValueError("Invalid value for cover: %s.  Choices are %s"
-                             % (cover, ",".join(cover_values)))
-
-        direction_values = ('children', 'parents')
-        if direction not in direction_values:
-            raise ValueError("Invalid value for direction: %s.  Choices are %s"
-                             % (direction, ",".join(direction_values)))
+        # Make sure kwargs have legal values; raise ValueError if not.
+        def validate(name, val, allowed_values):
+            if val not in allowed_values:
+                raise ValueError("Invalid value for %s: %s.  Choices are %s"
+                                 % (name, val, ",".join(allowed_values)))
+        validate('cover',     cover,     ('nodes', 'edges', 'paths'))
+        validate('direction', direction, ('children', 'parents'))
+        validate('order',     order,     ('pre', 'post'))
 
         if visited is None:
             visited = set()
-
-        result = (d, self) if depth else self
         key = key_fun(self)
 
-        if key in visited:
-            if cover == 'nodes':    return
-            if yield_root or d > 0: yield result
-            if cover == 'edges':    return
-        else:
-            if yield_root or d > 0: yield result
+        # Node traversal does not yield visited nodes.
+        if key in visited and cover == 'nodes':
+            return
 
-        successors = self.dependencies
-        if direction == 'parents':
-            successors = self.dependents
+        # Determine whether and what to yield for this node.
+        yield_me = yield_root or d > 0
+        result = (d, self) if depth else self
 
-        visited.add(key)
-        for name in sorted(successors):
-            child = successors[name]
-            for elt in child.preorder_traversal(visited, d+1, **kwargs):
-                yield elt
+        # Preorder traversal yields before successors
+        if yield_me and order == 'pre':
+            yield result
+
+        # Edge traversal yields but skips children of visited nodes
+        if not (key in visited and cover == 'edges'):
+            # This code determines direction and yields the children/parents
+            successors = self.dependencies
+            if direction == 'parents':
+                successors = self.dependents
+
+            visited.add(key)
+            for name in sorted(successors):
+                child = successors[name]
+                for elt in child.traverse(visited, d+1, **kwargs):
+                    yield elt
+
+        # Postorder traversal yields after successors
+        if yield_me and order == 'post':
+            yield result
 
 
     @property
@@ -540,13 +558,14 @@ def prefix(self):
 
 
     def dep_hash(self, length=None):
-        """Return a hash representing the dependencies of this spec
-           This will always normalize first so that the hash is consistent.
-        """
-        self.normalize()
+        """Return a hash representing all dependencies of this spec
+           (direct and indirect).
 
+           If you want this hash to be consistent, you should
+           concretize the spec first so that it is not ambiguous.
+        """
         sha = hashlib.sha1()
-        sha.update(str(self.dependencies))
+        sha.update(self.dep_string())
         full_hash = sha.hexdigest()
 
         return full_hash[:length]
@@ -609,7 +628,7 @@ def _expand_virtual_packages(self):
               a problem.
         """
         while True:
-            virtuals =[v for v in self.preorder_traversal() if v.virtual]
+            virtuals =[v for v in self.traverse() if v.virtual]
             if not virtuals:
                 return
 
@@ -620,7 +639,7 @@ def _expand_virtual_packages(self):
                 spec._replace_with(concrete)
 
             # If there are duplicate providers or duplicate provider deps, this
-            # consolidates them and merges constraints.
+            # consolidates them and merge constraints.
             self.normalize(force=True)
 
 
@@ -654,47 +673,51 @@ def concretized(self):
         return clone
 
 
-    def flat_dependencies(self):
-        """Return a DependencyMap containing all of this spec's dependencies
-           with their constraints merged.  If there are any conflicts, throw
-           an exception.
+    def flat_dependencies(self, **kwargs):
+        """Return a DependencyMap containing all of this spec's
+           dependencies with their constraints merged.
 
-           This will work even on specs that are not normalized; i.e. specs
-           that have two instances of the same dependency in the DAG.
-           This is used as the first step of normalization.
+           If copy is True, returns merged copies of its dependencies
+           without modifying the spec it's called on.
+
+           If copy is False, clears this spec's dependencies and
+           returns them.
         """
-        # This ensures that the package descriptions themselves are consistent
-        if not self.virtual:
-            self.package.validate_dependencies()
+        copy = kwargs.get('copy', True)
 
-        # Once that is guaranteed, we know any constraint violations are due
-        # to the spec -- so they're the user's fault, not Spack's.
         flat_deps = DependencyMap()
         try:
-            for spec in self.preorder_traversal():
+            for spec in self.traverse(root=False):
                 if spec.name not in flat_deps:
-                    new_spec = spec.copy(dependencies=False)
-                    flat_deps[spec.name] = new_spec
-
+                    if copy:
+                        flat_deps[spec.name] = spec.copy(deps=False)
+                    else:
+                        flat_deps[spec.name] = spec
                 else:
                     flat_deps[spec.name].constrain(spec)
 
-        except UnsatisfiableSpecError, e:
-            # This REALLY shouldn't happen unless something is wrong in spack.
-            # It means we got a spec DAG with two instances of the same package
-            # that had inconsistent constraints.  There's no way for a user to
-            # produce a spec like this (the parser adds all deps to the root),
-            # so this means OUR code is not sane!
-            raise InconsistentSpecError("Invalid Spec DAG: %s" % e.message)
+            if not copy:
+                for dep in flat_deps.values():
+                    dep.dependencies.clear()
+                    dep.dependents.clear()
+                self.dependencies.clear()
 
-        return flat_deps
+            return flat_deps
+
+        except UnsatisfiableSpecError, e:
+            # Here, the DAG contains two instances of the same package
+            # with inconsistent constraints.  Users cannot produce
+            # inconsistent specs like this on the command line: the
+            # parser doesn't allow it. Spack must be broken!
+            raise InconsistentSpecError("Invalid Spec DAG: %s" % e.message)
 
 
     def flatten(self):
         """Pull all dependencies up to the root (this spec).
            Merge constraints for dependencies with the same name, and if they
            conflict, throw an exception. """
-        self.dependencies = self.flat_dependencies()
+        for dep in self.flat_dependencies(copy=False):
+            self._add_dependency(dep)
 
 
     def _normalize_helper(self, visited, spec_deps, provider_index):
@@ -797,11 +820,12 @@ def normalize(self, **kwargs):
         # Ensure first that all packages & compilers in the DAG exist.
         self.validate_names()
 
-        # Then ensure that the packages referenced are sane, that the
-        # provided spec is sane, and that all dependency specs are in the
-        # root node of the spec.  flat_dependencies will do this for us.
-        spec_deps = self.flat_dependencies()
-        self.dependencies.clear()
+        # Ensure that the package & dep descriptions are consistent & sane
+        if not self.virtual:
+            self.package.validate_dependencies()
+
+        # Get all the dependencies into one DependencyMap
+        spec_deps = self.flat_dependencies(copy=False)
 
         # Figure out which of the user-provided deps provide virtual deps.
         # Remove virtual deps that are already provided by something in the spec
@@ -843,7 +867,7 @@ def validate_names(self):
            If they're not, it will raise either UnknownPackageError or
            UnsupportedCompilerError.
         """
-        for spec in self.preorder_traversal():
+        for spec in self.traverse():
             # Don't get a package for a virtual name.
             if not spec.virtual:
                 spack.db.get(spec.name)
@@ -911,17 +935,17 @@ def _constrain_dependencies(self, other):
     def common_dependencies(self, other):
         """Return names of dependencies that self an other have in common."""
         common = set(
-            s.name for s in self.preorder_traversal(root=False))
+            s.name for s in self.traverse(root=False))
         common.intersection_update(
-            s.name for s in other.preorder_traversal(root=False))
+            s.name for s in other.traverse(root=False))
         return common
 
 
     def dep_difference(self, other):
         """Returns dependencies in self that are not in other."""
-        mine = set(s.name for s in self.preorder_traversal(root=False))
+        mine = set(s.name for s in self.traverse(root=False))
         mine.difference_update(
-            s.name for s in other.preorder_traversal(root=False))
+            s.name for s in other.traverse(root=False))
         return mine
 
 
@@ -980,8 +1004,8 @@ def satisfies_dependencies(self, other):
                 return False
 
         # For virtual dependencies, we need to dig a little deeper.
-        self_index = ProviderIndex(self.preorder_traversal(), restrict=True)
-        other_index = ProviderIndex(other.preorder_traversal(), restrict=True)
+        self_index = ProviderIndex(self.traverse(), restrict=True)
+        other_index = ProviderIndex(other.traverse(), restrict=True)
 
         # This handles cases where there are already providers for both vpkgs
         if not self_index.satisfies(other_index):
@@ -1003,7 +1027,7 @@ def satisfies_dependencies(self, other):
 
     def virtual_dependencies(self):
         """Return list of any virtual deps in this spec."""
-        return [spec for spec in self.preorder_traversal() if spec.virtual]
+        return [spec for spec in self.traverse() if spec.virtual]
 
 
     def _dup(self, other, **kwargs):
@@ -1018,22 +1042,29 @@ def _dup(self, other, **kwargs):
                Whether deps should be copied too.  Set to false to copy a
                spec but not its dependencies.
         """
-        # TODO: this needs to handle DAGs.
+        # Local node attributes get copied first.
         self.name = other.name
         self.versions = other.versions.copy()
         self.variants = other.variants.copy()
         self.architecture = other.architecture
-        self.compiler = None
-        if other.compiler:
-            self.compiler = other.compiler.copy()
-
+        self.compiler = other.compiler.copy() if other.compiler else None
         self.dependents = DependencyMap()
-        copy_deps = kwargs.get('dependencies', True)
-        if copy_deps:
-            self.dependencies = other.dependencies.copy()
-        else:
-            self.dependencies = DependencyMap()
+        self.dependencies = DependencyMap()
 
+        # If we copy dependencies, preserve DAG structure in the new spec
+        if kwargs.get('deps', True):
+            # This copies the deps from other using _dup(deps=False)
+            new_nodes = other.flat_dependencies()
+            new_nodes[self.name] = self
+
+            # Hook everything up properly here by traversing.
+            for spec in other.traverse(cover='nodes'):
+                parent = new_nodes[spec.name]
+                for child in spec.dependencies:
+                    if child not in parent.dependencies:
+                        parent._add_dependency(new_nodes[child])
+
+        # Since we preserved structure, we can copy _normal safely.
         self._normal = other._normal
         self._concrete = other._concrete
 
@@ -1057,7 +1088,7 @@ def version(self):
 
     def __getitem__(self, name):
         """TODO: reconcile __getitem__, _add_dependency, __contains__"""
-        for spec in self.preorder_traversal():
+        for spec in self.traverse():
             if spec.name == name:
                 return spec
 
@@ -1068,15 +1099,82 @@ def __contains__(self, spec):
         """True if this spec has any dependency that satisfies the supplied
            spec."""
         spec = self._autospec(spec)
-        for s in self.preorder_traversal():
+        for s in self.traverse():
             if s.satisfies(spec):
                 return True
         return False
 
 
-    def _cmp_key(self):
+    def sorted_deps(self):
+        """Return a list of all dependencies sorted by name."""
+        deps = self.flat_dependencies()
+        return tuple(deps[name] for name in sorted(deps))
+
+
+    def _eq_dag(self, other, vs, vo):
+        """Recursive helper for eq_dag and ne_dag.  Does the actual DAG
+           traversal."""
+        vs.add(id(self))
+        vo.add(id(other))
+
+        if self.ne_node(other):
+            return False
+
+        if len(self.dependencies) != len(other.dependencies):
+            return False
+
+        ssorted = [self.dependencies[name]  for name in sorted(self.dependencies)]
+        osorted = [other.dependencies[name] for name in sorted(other.dependencies)]
+
+        for s, o in zip(ssorted, osorted):
+            visited_s = id(s) in vs
+            visited_o = id(o) in vo
+
+            # Check for duplicate or non-equal dependencies
+            if visited_s != visited_o: return False
+
+            # Skip visited nodes
+            if visited_s or visited_o: continue
+
+            # Recursive check for equality
+            if not s._eq_dag(o, vs, vo):
+                return False
+
+        return True
+
+
+    def eq_dag(self, other):
+        """True if the full dependency DAGs of specs are equal"""
+        return self._eq_dag(other, set(), set())
+
+
+    def ne_dag(self, other):
+        """True if the full dependency DAGs of specs are not equal"""
+        return not self.eq_dag(other)
+
+
+    def _cmp_node(self):
+        """Comparison key for just *this node* and not its deps."""
         return (self.name, self.versions, self.variants,
-                self.architecture, self.compiler, self.dependencies)
+                self.architecture, self.compiler)
+
+
+    def eq_node(self, other):
+        """Equality with another spec, not including dependencies."""
+        return self._cmp_node() == other._cmp_node()
+
+
+    def ne_node(self, other):
+        """Inequality with another spec, not including dependencies."""
+        return self._cmp_node() != other._cmp_node()
+
+
+    def _cmp_key(self):
+        """Comparison key for this node and all dependencies *without*
+           considering structure.  This is the default, as
+           normalization will restore structure.
+        """
+        return self._cmp_node() + (self.sorted_deps(),)
 
 
     def colorized(self):
@@ -1179,12 +1277,12 @@ def write(s, c):
         return result
 
 
+    def dep_string(self):
+        return ''.join("^" + dep.format() for dep in self.sorted_deps())
+
+
     def __str__(self):
-        by_name = lambda d: d.name
-        deps = self.preorder_traversal(key=by_name, root=False)
-        sorted_deps = sorted(deps, key=by_name)
-        dep_string = ''.join("^" + dep.format() for dep in sorted_deps)
-        return self.format() + dep_string
+        return self.format() + self.dep_string()
 
 
     def tree(self, **kwargs):
@@ -1200,7 +1298,7 @@ def tree(self, **kwargs):
         out = ""
         cur_id = 0
         ids = {}
-        for d, node in self.preorder_traversal(cover=cover, depth=True):
+        for d, node in self.traverse(order='pre', cover=cover, depth=True):
             out += " " * indent
             if depth:
                 out += "%-4d" % d
diff --git a/lib/spack/spack/test/__init__.py b/lib/spack/spack/test/__init__.py
index 5442189c2e..c2dfc51aa3 100644
--- a/lib/spack/spack/test/__init__.py
+++ b/lib/spack/spack/test/__init__.py
@@ -45,7 +45,8 @@
               'multimethod',
               'install',
               'package_sanity',
-              'config']
+              'config',
+              'directory_layout']
 
 
 def list_tests():
diff --git a/lib/spack/spack/test/directory_layout.py b/lib/spack/spack/test/directory_layout.py
new file mode 100644
index 0000000000..3e52954cfe
--- /dev/null
+++ b/lib/spack/spack/test/directory_layout.py
@@ -0,0 +1,155 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""\
+This test verifies that the Spack directory layout works properly.
+"""
+import unittest
+import tempfile
+import shutil
+import os
+from contextlib import closing
+
+from llnl.util.filesystem import *
+
+import spack
+from spack.spec import Spec
+from spack.packages import PackageDB
+from spack.directory_layout import SpecHashDirectoryLayout
+
+class DirectoryLayoutTest(unittest.TestCase):
+    """Tests that a directory layout works correctly and produces a
+       consistent install path."""
+
+    def setUp(self):
+        self.tmpdir = tempfile.mkdtemp()
+        self.layout = SpecHashDirectoryLayout(self.tmpdir)
+
+
+    def tearDown(self):
+        shutil.rmtree(self.tmpdir, ignore_errors=True)
+        self.layout = None
+
+
+    def test_read_and_write_spec(self):
+        """This goes through each package in spack and creates a directory for
+           it.  It then ensures that the spec for the directory's
+           installed package can be read back in consistently, and
+           finally that the directory can be removed by the directory
+           layout.
+        """
+        for pkg in spack.db.all_packages():
+            spec = pkg.spec
+
+            # If a spec fails to concretize, just skip it.  If it is a
+            # real error, it will be caught by concretization tests.
+            try:
+                spec.concretize()
+            except:
+                continue
+
+            self.layout.make_path_for_spec(spec)
+
+            install_dir = self.layout.path_for_spec(spec)
+            spec_path = self.layout.spec_file_path(spec)
+
+            # Ensure directory has been created in right place.
+            self.assertTrue(os.path.isdir(install_dir))
+            self.assertTrue(install_dir.startswith(self.tmpdir))
+
+            # Ensure spec file exists when directory is created
+            self.assertTrue(os.path.isfile(spec_path))
+            self.assertTrue(spec_path.startswith(install_dir))
+
+            # Make sure spec file can be read back in to get the original spec
+            spec_from_file = self.layout.read_spec(spec_path)
+            self.assertEqual(spec, spec_from_file)
+            self.assertTrue(spec.eq_dag, spec_from_file)
+            self.assertTrue(spec_from_file.concrete)
+
+            # Ensure that specs that come out "normal" are really normal.
+            with closing(open(spec_path)) as spec_file:
+                read_separately = Spec(spec_file.read())
+
+                read_separately.normalize()
+                self.assertEqual(read_separately, spec_from_file)
+
+                read_separately.concretize()
+                self.assertEqual(read_separately, spec_from_file)
+
+            # Make sure the dep hash of the read-in spec is the same
+            self.assertEqual(spec.dep_hash(), spec_from_file.dep_hash())
+
+            # Ensure directories are properly removed
+            self.layout.remove_path_for_spec(spec)
+            self.assertFalse(os.path.isdir(install_dir))
+            self.assertFalse(os.path.exists(install_dir))
+
+
+    def test_handle_unknown_package(self):
+        """This test ensures that spack can at least do *some*
+           operations with packages that are installed but that it
+           does not know about.  This is actually not such an uncommon
+           scenario with spack; it can happen when you switch from a
+           git branch where you're working on a new package.
+
+           This test ensures that the directory layout stores enough
+           information about installed packages' specs to uninstall
+           or query them again if the package goes away.
+        """
+        mock_db = PackageDB(spack.mock_packages_path)
+
+        not_in_mock = set(spack.db.all_package_names()).difference(
+            set(mock_db.all_package_names()))
+
+        # Create all the packages that are not in mock.
+        installed_specs = {}
+        for pkg_name in not_in_mock:
+            spec = spack.db.get(pkg_name).spec
+
+            # If a spec fails to concretize, just skip it.  If it is a
+            # real error, it will be caught by concretization tests.
+            try:
+                spec.concretize()
+            except:
+                continue
+
+            self.layout.make_path_for_spec(spec)
+            installed_specs[spec] = self.layout.path_for_spec(spec)
+
+        tmp = spack.db
+        spack.db = mock_db
+
+        # Now check that even without the package files, we know
+        # enough to read a spec from the spec file.
+        for spec, path in installed_specs.items():
+            spec_from_file = self.layout.read_spec(join_path(path, '.spec'))
+
+            # To satisfy these conditions, directory layouts need to
+            # read in concrete specs from their install dirs somehow.
+            self.assertEqual(path, self.layout.path_for_spec(spec_from_file))
+            self.assertEqual(spec, spec_from_file)
+            self.assertEqual(spec.dep_hash(), spec_from_file.dep_hash())
+
+        spack.db = tmp
diff --git a/lib/spack/spack/test/install.py b/lib/spack/spack/test/install.py
index a92bd92289..8047ab92e3 100644
--- a/lib/spack/spack/test/install.py
+++ b/lib/spack/spack/test/install.py
@@ -25,15 +25,18 @@
 import os
 import unittest
 import shutil
+import tempfile
 from contextlib import closing
 
 from llnl.util.filesystem import *
 
 import spack
 from spack.stage import Stage
+from spack.directory_layout import SpecHashDirectoryLayout
 from spack.util.executable import which
 from spack.test.mock_packages_test import *
 
+
 dir_name = 'trivial-1.0'
 archive_name = 'trivial-1.0.tar.gz'
 install_test_package = 'trivial_install_test_package'
@@ -66,9 +69,16 @@ def setUp(self):
             tar = which('tar')
             tar('-czf', archive_name, dir_name)
 
-        # We use a fake pacakge, so skip the checksum.
+        # We use a fake package, so skip the checksum.
         spack.do_checksum = False
 
+        # Use a fake install directory to avoid conflicts bt/w
+        # installed pkgs and mock packages.
+        self.tmpdir = tempfile.mkdtemp()
+        self.orig_layout = spack.install_layout
+        spack.install_layout = SpecHashDirectoryLayout(self.tmpdir)
+
+
     def tearDown(self):
         super(InstallTest, self).tearDown()
 
@@ -78,6 +88,10 @@ def tearDown(self):
         # Turn checksumming back on
         spack.do_checksum = True
 
+        # restore spack's layout.
+        spack.install_layout = self.orig_layout
+        shutil.rmtree(self.tmpdir, ignore_errors=True)
+
 
     def test_install_and_uninstall(self):
         # Get a basic concrete spec for the trivial install package.
diff --git a/lib/spack/spack/test/spec_dag.py b/lib/spack/spack/test/spec_dag.py
index 0c0b214ab7..322f34cf02 100644
--- a/lib/spack/spack/test/spec_dag.py
+++ b/lib/spack/spack/test/spec_dag.py
@@ -48,7 +48,7 @@ def test_conflicting_package_constraints(self):
                           spec.package.validate_dependencies)
 
 
-    def test_unique_node_traversal(self):
+    def test_preorder_node_traversal(self):
         dag = Spec('mpileaks ^zmpi')
         dag.normalize()
 
@@ -56,14 +56,14 @@ def test_unique_node_traversal(self):
                  'zmpi', 'fake']
         pairs = zip([0,1,2,3,4,2,3], names)
 
-        traversal = dag.preorder_traversal()
+        traversal = dag.traverse()
         self.assertListEqual([x.name for x in traversal], names)
 
-        traversal = dag.preorder_traversal(depth=True)
+        traversal = dag.traverse(depth=True)
         self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
 
 
-    def test_unique_edge_traversal(self):
+    def test_preorder_edge_traversal(self):
         dag = Spec('mpileaks ^zmpi')
         dag.normalize()
 
@@ -71,14 +71,14 @@ def test_unique_edge_traversal(self):
                  'libelf', 'zmpi', 'fake', 'zmpi']
         pairs = zip([0,1,2,3,4,3,2,3,1], names)
 
-        traversal = dag.preorder_traversal(cover='edges')
+        traversal = dag.traverse(cover='edges')
         self.assertListEqual([x.name for x in traversal], names)
 
-        traversal = dag.preorder_traversal(cover='edges', depth=True)
+        traversal = dag.traverse(cover='edges', depth=True)
         self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
 
 
-    def test_unique_path_traversal(self):
+    def test_preorder_path_traversal(self):
         dag = Spec('mpileaks ^zmpi')
         dag.normalize()
 
@@ -86,10 +86,55 @@ def test_unique_path_traversal(self):
                  'libelf', 'zmpi', 'fake', 'zmpi', 'fake']
         pairs = zip([0,1,2,3,4,3,2,3,1,2], names)
 
-        traversal = dag.preorder_traversal(cover='paths')
+        traversal = dag.traverse(cover='paths')
         self.assertListEqual([x.name for x in traversal], names)
 
-        traversal = dag.preorder_traversal(cover='paths', depth=True)
+        traversal = dag.traverse(cover='paths', depth=True)
+        self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
+
+
+    def test_postorder_node_traversal(self):
+        dag = Spec('mpileaks ^zmpi')
+        dag.normalize()
+
+        names = ['libelf', 'libdwarf', 'dyninst', 'fake', 'zmpi',
+                 'callpath', 'mpileaks']
+        pairs = zip([4,3,2,3,2,1,0], names)
+
+        traversal = dag.traverse(order='post')
+        self.assertListEqual([x.name for x in traversal], names)
+
+        traversal = dag.traverse(depth=True, order='post')
+        self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
+
+
+    def test_postorder_edge_traversal(self):
+        dag = Spec('mpileaks ^zmpi')
+        dag.normalize()
+
+        names = ['libelf', 'libdwarf', 'libelf', 'dyninst', 'fake', 'zmpi',
+                 'callpath', 'zmpi', 'mpileaks']
+        pairs = zip([4,3,3,2,3,2,1,1,0], names)
+
+        traversal = dag.traverse(cover='edges', order='post')
+        self.assertListEqual([x.name for x in traversal], names)
+
+        traversal = dag.traverse(cover='edges', depth=True, order='post')
+        self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
+
+
+    def test_postorder_path_traversal(self):
+        dag = Spec('mpileaks ^zmpi')
+        dag.normalize()
+
+        names = ['libelf', 'libdwarf', 'libelf', 'dyninst', 'fake', 'zmpi',
+                 'callpath', 'fake', 'zmpi', 'mpileaks']
+        pairs = zip([4,3,3,2,3,2,1,2,1,0], names)
+
+        traversal = dag.traverse(cover='paths', order='post')
+        self.assertListEqual([x.name for x in traversal], names)
+
+        traversal = dag.traverse(cover='paths', depth=True, order='post')
         self.assertListEqual([(x, y.name) for x,y in traversal], pairs)
 
 
@@ -142,7 +187,7 @@ def test_normalize_with_virtual_spec(self):
 
         # make sure nothing with the same name occurs twice
         counts = {}
-        for spec in dag.preorder_traversal(keyfun=id):
+        for spec in dag.traverse(key=id):
             if not spec.name in counts:
                 counts[spec.name] = 0
             counts[spec.name] += 1
@@ -152,7 +197,7 @@ def test_normalize_with_virtual_spec(self):
 
 
     def check_links(self, spec_to_check):
-        for spec in spec_to_check.preorder_traversal():
+        for spec in spec_to_check.traverse():
             for dependent in spec.dependents.values():
                 self.assertIn(
                     spec.name, dependent.dependencies,
@@ -221,30 +266,53 @@ def test_invalid_dep(self):
 
 
     def test_equal(self):
-        spec = Spec('mpileaks ^callpath ^libelf ^libdwarf')
-        self.assertNotEqual(spec, Spec(
-            'mpileaks', Spec('callpath',
-                             Spec('libdwarf',
-                                  Spec('libelf')))))
-        self.assertNotEqual(spec, Spec(
-            'mpileaks', Spec('callpath',
-                             Spec('libelf',
-                                  Spec('libdwarf')))))
+        # Different spec structures to test for equality
+        flat = Spec('mpileaks ^callpath ^libelf ^libdwarf')
 
-        self.assertEqual(spec, Spec(
-            'mpileaks', Spec('callpath'), Spec('libdwarf'), Spec('libelf')))
+        flat_init = Spec(
+            'mpileaks', Spec('callpath'), Spec('libdwarf'), Spec('libelf'))
 
-        self.assertEqual(spec, Spec(
-            'mpileaks', Spec('libelf'), Spec('libdwarf'), Spec('callpath')))
+        flip_flat = Spec(
+            'mpileaks', Spec('libelf'), Spec('libdwarf'), Spec('callpath'))
+
+        dag = Spec('mpileaks', Spec('callpath',
+                                    Spec('libdwarf',
+                                         Spec('libelf'))))
+
+        flip_dag = Spec('mpileaks', Spec('callpath',
+                                         Spec('libelf',
+                                              Spec('libdwarf'))))
+
+        # All these are equal to each other with regular ==
+        specs = (flat, flat_init, flip_flat, dag, flip_dag)
+        for lhs, rhs in zip(specs, specs):
+            self.assertEqual(lhs, rhs)
+            self.assertEqual(str(lhs), str(rhs))
+
+        # Same DAGs constructed different ways are equal
+        self.assertTrue(flat.eq_dag(flat_init))
+
+        # order at same level does not matter -- (dep on same parent)
+        self.assertTrue(flat.eq_dag(flip_flat))
+
+        # DAGs should be unequal if nesting is different
+        self.assertFalse(flat.eq_dag(dag))
+        self.assertFalse(flat.eq_dag(flip_dag))
+        self.assertFalse(flip_flat.eq_dag(dag))
+        self.assertFalse(flip_flat.eq_dag(flip_dag))
+        self.assertFalse(dag.eq_dag(flip_dag))
 
 
     def test_normalize_mpileaks(self):
+        # Spec parsed in from a string
         spec = Spec('mpileaks ^mpich ^callpath ^dyninst ^libelf@1.8.11 ^libdwarf')
 
+        # What that spec should look like after parsing
         expected_flat = Spec(
             'mpileaks', Spec('mpich'), Spec('callpath'), Spec('dyninst'),
             Spec('libelf@1.8.11'), Spec('libdwarf'))
 
+        # What it should look like after normalization
         mpich = Spec('mpich')
         libelf = Spec('libelf@1.8.11')
         expected_normalized = Spec(
@@ -257,7 +325,10 @@ def test_normalize_mpileaks(self):
                  mpich),
             mpich)
 
-        expected_non_unique_nodes = Spec(
+        # Similar to normalized spec, but now with copies of the same
+        # libelf node.  Normalization should result in a single unique
+        # node for each package, so this is the wrong DAG.
+        non_unique_nodes = Spec(
             'mpileaks',
             Spec('callpath',
                  Spec('dyninst',
@@ -267,21 +338,33 @@ def test_normalize_mpileaks(self):
                  mpich),
             Spec('mpich'))
 
-        self.assertEqual(expected_normalized, expected_non_unique_nodes)
-
-        self.assertEqual(str(expected_normalized), str(expected_non_unique_nodes))
-        self.assertEqual(str(spec), str(expected_non_unique_nodes))
-        self.assertEqual(str(expected_normalized), str(spec))
+        # All specs here should be equal under regular equality
+        specs = (spec, expected_flat, expected_normalized, non_unique_nodes)
+        for lhs, rhs in zip(specs, specs):
+            self.assertEqual(lhs, rhs)
+            self.assertEqual(str(lhs), str(rhs))
 
+        # Test that equal and equal_dag are doing the right thing
         self.assertEqual(spec, expected_flat)
-        self.assertNotEqual(spec, expected_normalized)
-        self.assertNotEqual(spec, expected_non_unique_nodes)
+        self.assertTrue(spec.eq_dag(expected_flat))
+
+        self.assertEqual(spec, expected_normalized)
+        self.assertFalse(spec.eq_dag(expected_normalized))
+
+        self.assertEqual(spec, non_unique_nodes)
+        self.assertFalse(spec.eq_dag(non_unique_nodes))
 
         spec.normalize()
 
-        self.assertNotEqual(spec, expected_flat)
+        # After normalizing, spec_dag_equal should match the normalized spec.
+        self.assertEqual(spec, expected_flat)
+        self.assertFalse(spec.eq_dag(expected_flat))
+
         self.assertEqual(spec, expected_normalized)
-        self.assertEqual(spec, expected_non_unique_nodes)
+        self.assertTrue(spec.eq_dag(expected_normalized))
+
+        self.assertEqual(spec, non_unique_nodes)
+        self.assertFalse(spec.eq_dag(non_unique_nodes))
 
 
     def test_normalize_with_virtual_package(self):
@@ -309,3 +392,56 @@ def test_contains(self):
         self.assertIn(Spec('libdwarf'), spec)
         self.assertNotIn(Spec('libgoblin'), spec)
         self.assertIn(Spec('mpileaks'), spec)
+
+
+    def test_copy_simple(self):
+        orig = Spec('mpileaks')
+        copy = orig.copy()
+
+        self.check_links(copy)
+
+        self.assertEqual(orig, copy)
+        self.assertTrue(orig.eq_dag(copy))
+        self.assertEqual(orig._normal, copy._normal)
+        self.assertEqual(orig._concrete, copy._concrete)
+
+        # ensure no shared nodes bt/w orig and copy.
+        orig_ids = set(id(s) for s in orig.traverse())
+        copy_ids = set(id(s) for s in copy.traverse())
+        self.assertFalse(orig_ids.intersection(copy_ids))
+
+
+    def test_copy_normalized(self):
+        orig = Spec('mpileaks')
+        orig.normalize()
+        copy = orig.copy()
+
+        self.check_links(copy)
+
+        self.assertEqual(orig, copy)
+        self.assertTrue(orig.eq_dag(copy))
+        self.assertEqual(orig._normal, copy._normal)
+        self.assertEqual(orig._concrete, copy._concrete)
+
+        # ensure no shared nodes bt/w orig and copy.
+        orig_ids = set(id(s) for s in orig.traverse())
+        copy_ids = set(id(s) for s in copy.traverse())
+        self.assertFalse(orig_ids.intersection(copy_ids))
+
+
+    def test_copy_concretized(self):
+        orig = Spec('mpileaks')
+        orig.concretize()
+        copy = orig.copy()
+
+        self.check_links(copy)
+
+        self.assertEqual(orig, copy)
+        self.assertTrue(orig.eq_dag(copy))
+        self.assertEqual(orig._normal, copy._normal)
+        self.assertEqual(orig._concrete, copy._concrete)
+
+        # ensure no shared nodes bt/w orig and copy.
+        orig_ids = set(id(s) for s in orig.traverse())
+        copy_ids = set(id(s) for s in copy.traverse())
+        self.assertFalse(orig_ids.intersection(copy_ids))