diff --git a/pkg/blobserver/google/drive/service/service.go b/pkg/blobserver/google/drive/service/service.go
index 8cbd5ca70..2ea062ecc 100644
--- a/pkg/blobserver/google/drive/service/service.go
+++ b/pkg/blobserver/google/drive/service/service.go
@@ -24,6 +24,7 @@ import (
 	"io"
 	"math"
 	"net/http"
+	"os"
 
 	"camlistore.org/third_party/code.google.com/p/goauth2/oauth"
 	client "camlistore.org/third_party/code.google.com/p/google-api-go-client/drive/v2"
@@ -42,28 +43,38 @@ type DriveService struct {
 	parentId   string
 }
 
-// New initiates a new DriveService.
+// New initiates a new DriveService. parentId is the ID of the directory
+// that will be used as the current directory in methods on the returned
+// DriveService (such as Get). If empty, it defaults to the root of the
+// drive.
 func New(transport *oauth.Transport, parentId string) (*DriveService, error) {
 	apiservice, err := client.New(transport.Client())
 	if err != nil {
 		return nil, err
 	}
+	if parentId == "" {
+		// because "root" is known as a special alias for the root directory in drive.
+		parentId = "root"
+	}
 	service := &DriveService{transport: transport, apiservice: apiservice, parentId: parentId}
 	return service, err
 }
 
-// Get retrieves a file with its title
+// Get retrieves a file with its title equal to the provided id and a child of
+// the parentId as given to New. If not found, os.ErrNotExist is returned.
 func (s *DriveService) Get(id string) (*client.File, error) {
 	req := s.apiservice.Files.List()
 	// TODO: use field selectors
 	query := fmt.Sprintf("'%s' in parents and title = '%s'", s.parentId, id)
 	req.Q(query)
 	files, err := req.Do()
-
-	if err != nil || len(files.Items) < 1 {
+	if err != nil {
 		return nil, err
 	}
-	return files.Items[0], err
+	if len(files.Items) < 1 {
+		return nil, os.ErrNotExist
+	}
+	return files.Items[0], nil
 }
 
 // Lists the folder identified by parentId.
@@ -104,14 +115,25 @@ func (s *DriveService) Upsert(id string, data io.Reader) (file *client.File, err
 	return s.apiservice.Files.Update(file.Id, file).Media(data).Do()
 }
 
+var errNoDownload = errors.New("file can not be downloaded directly (conversion needed?)")
+
 // Fetch retrieves the metadata and contents of a file.
 func (s *DriveService) Fetch(id string) (body io.ReadCloser, size uint32, err error) {
 	file, err := s.Get(id)
-
+	if err != nil {
+		return
+	}
 	// TODO: maybe in the case of no download link, remove the file.
 	// The file should have malformed or converted to a Docs file
 	// unwantedly.
-	if err != nil || file == nil || file.DownloadUrl != "" {
+	// TODO(mpl): I do not think the above comment is accurate. It
+	// looks like at least one case we do not get a DownloadUrl is when
+	// the UI would make you pick a conversion format first (spreadsheet,
+	// doc, etc). -> we should see if the API offers the possibility to do
+	// that conversion. and we could pass the type(s) we want (pdf, xls, doc...)
+	// as arguments (in an options struct) to Fetch.
+	if file.DownloadUrl == "" {
+		err = errNoDownload
 		return
 	}
 
diff --git a/pkg/index/corpus.go b/pkg/index/corpus.go
index a5476b9b9..779dbd81b 100644
--- a/pkg/index/corpus.go
+++ b/pkg/index/corpus.go
@@ -104,6 +104,9 @@ type Corpus struct {
 
 	mediaTags map[blob.Ref]map[string]string // wholeref -> "album" -> "foo"
 
+	permanodesByTime    *lazySortedPermanodes // cache of permanodes sorted by creation time.
+	permanodesByModtime *lazySortedPermanodes // cache of permanodes sorted by modtime.
+
 	// scratch string slice
 	ss []string
 }
@@ -146,7 +149,7 @@ type PermanodeMeta struct {
 }
 
 func newCorpus() *Corpus {
-	return &Corpus{
+	c := &Corpus{
 		blobs:        make(map[blob.Ref]*camtypes.BlobMeta),
 		camBlobs:     make(map[string]map[blob.Ref]*camtypes.BlobMeta),
 		files:        make(map[blob.Ref]camtypes.FileInfo),
@@ -161,6 +164,15 @@ func newCorpus() *Corpus {
 		deletes:      make(map[blob.Ref][]deletion),
 		claimBack:    make(map[blob.Ref][]*camtypes.Claim),
 	}
+	c.permanodesByModtime = &lazySortedPermanodes{
+		c:      c,
+		pnTime: c.PermanodeModtimeLocked,
+	}
+	c.permanodesByTime = &lazySortedPermanodes{
+		c:      c,
+		pnTime: c.PermanodeAnyTimeLocked,
+	}
+	return c
 }
 
 func NewCorpusFromStorage(s sorted.KeyValue) (*Corpus, error) {
@@ -708,16 +720,73 @@ func (s byPermanodeTime) Less(i, j int) bool {
 	return s[i].t.Before(s[j].t)
 }
 
-func (c *Corpus) permanodesByModtimeLocked() []pnAndTime {
-	pns := make([]pnAndTime, 0, len(c.permanodes))
-	for pn := range c.permanodes {
-		if c.IsDeletedLocked(pn) {
-			continue
+type lazySortedPermanodes struct {
+	c      *Corpus
+	pnTime func(blob.Ref) (time.Time, bool) // returns permanode's time (if any) to sort on
+
+	mu                  sync.Mutex  // guards sortedCache and ofGen
+	sortedCache         []pnAndTime // nil if invalidated
+	sortedCacheReversed []pnAndTime // nil if invalidated
+	ofGen               int64       // the Corpus.gen from which sortedCache was built
+}
+
+func reversedCopy(original []pnAndTime) []pnAndTime {
+	l := len(original)
+	reversed := make([]pnAndTime, l)
+	for k, v := range original {
+		reversed[l-1-k] = v
+	}
+	return reversed
+}
+
+// The Corpus must already be locked with RLock.
+func (lsp *lazySortedPermanodes) sorted(reverse bool) []pnAndTime {
+	lsp.mu.Lock()
+	defer lsp.mu.Unlock()
+	if lsp.ofGen == lsp.c.gen {
+		// corpus hasn't changed -> caches are still valid, if they exist.
+		if reverse {
+			if lsp.sortedCacheReversed != nil {
+				return lsp.sortedCacheReversed
+			}
+			if lsp.sortedCache != nil {
+				// using sortedCache to quickly build sortedCacheReversed
+				lsp.sortedCacheReversed = reversedCopy(lsp.sortedCache)
+				return lsp.sortedCacheReversed
+			}
 		}
-		if modt, ok := c.PermanodeModtimeLocked(pn); ok {
-			pns = append(pns, pnAndTime{pn, modt})
+		if !reverse {
+			if lsp.sortedCache != nil {
+				return lsp.sortedCache
+			}
+			if lsp.sortedCacheReversed != nil {
+				// using sortedCacheReversed to quickly build sortedCache
+				lsp.sortedCache = reversedCopy(lsp.sortedCacheReversed)
+				return lsp.sortedCache
+			}
 		}
 	}
+	// invalidate the caches
+	lsp.sortedCache = nil
+	lsp.sortedCacheReversed = nil
+	pns := make([]pnAndTime, 0, len(lsp.c.permanodes))
+	for pn := range lsp.c.permanodes {
+		if lsp.c.IsDeletedLocked(pn) {
+			continue
+		}
+		if pt, ok := lsp.pnTime(pn); ok {
+			pns = append(pns, pnAndTime{pn, pt})
+		}
+	}
+	// and rebuild one of them
+	if reverse {
+		sort.Sort(sort.Reverse(byPermanodeTime(pns)))
+		lsp.sortedCacheReversed = pns
+	} else {
+		sort.Sort(byPermanodeTime(pns))
+		lsp.sortedCache = pns
+	}
+	lsp.ofGen = lsp.c.gen
 	return pns
 }
 
@@ -745,23 +814,7 @@ func (c *Corpus) sendPermanodes(ctx *context.Context, ch chan<- camtypes.BlobMet
 func (c *Corpus) EnumeratePermanodesLastModifiedLocked(ctx *context.Context, ch chan<- camtypes.BlobMeta) error {
 	defer close(ch)
 
-	pns := c.permanodesByModtimeLocked()
-	sort.Sort(sort.Reverse(byPermanodeTime(pns)))
-	return c.sendPermanodes(ctx, ch, pns)
-}
-
-func (c *Corpus) permanodesByTimeLocked() []pnAndTime {
-	// TODO: cache this
-	pns := make([]pnAndTime, 0, len(c.permanodes))
-	for pn := range c.permanodes {
-		if c.IsDeletedLocked(pn) {
-			continue
-		}
-		if pt, ok := c.PermanodeAnyTimeLocked(pn); ok {
-			pns = append(pns, pnAndTime{pn, pt})
-		}
-	}
-	return pns
+	return c.sendPermanodes(ctx, ch, c.permanodesByModtime.sorted(true))
 }
 
 // EnumeratePermanodesCreatedLocked sends all permanodes to ch, or until ctx is done.
@@ -772,14 +825,7 @@ func (c *Corpus) permanodesByTimeLocked() []pnAndTime {
 func (c *Corpus) EnumeratePermanodesCreatedLocked(ctx *context.Context, ch chan<- camtypes.BlobMeta, newestFirst bool) error {
 	defer close(ch)
 
-	pns := c.permanodesByTimeLocked()
-	if newestFirst {
-		sort.Sort(sort.Reverse(byPermanodeTime(pns)))
-	} else {
-		sort.Sort(byPermanodeTime(pns))
-	}
-
-	return c.sendPermanodes(ctx, ch, pns)
+	return c.sendPermanodes(ctx, ch, c.permanodesByTime.sorted(newestFirst))
 }
 
 func (c *Corpus) GetBlobMeta(br blob.Ref) (camtypes.BlobMeta, error) {
diff --git a/pkg/index/corpus_test.go b/pkg/index/corpus_test.go
index 2b91d150d..8227a0aff 100644
--- a/pkg/index/corpus_test.go
+++ b/pkg/index/corpus_test.go
@@ -17,6 +17,7 @@ limitations under the License.
 package index_test
 
 import (
+	"fmt"
 	"reflect"
 	"testing"
 	"time"
@@ -339,3 +340,146 @@ func testEnumerateOrder(t *testing.T,
 		}
 	}
 }
+
+// should be run with -race
+func TestCacheSortedPermanodes_ModtimeRace(t *testing.T) {
+	testCacheSortedPermanodesRace(t,
+		func(c *index.Corpus, ctx *context.Context, ch chan<- camtypes.BlobMeta) error {
+			return c.EnumeratePermanodesLastModifiedLocked(ctx, ch)
+		},
+	)
+}
+
+// should be run with -race
+func TestCacheSortedPermanodes_CreateTimeRace(t *testing.T) {
+	testCacheSortedPermanodesRace(t,
+		func(c *index.Corpus, ctx *context.Context, ch chan<- camtypes.BlobMeta) error {
+			return c.EnumeratePermanodesCreatedLocked(ctx, ch, true)
+		},
+	)
+}
+
+func testCacheSortedPermanodesRace(t *testing.T,
+	enumFunc func(*index.Corpus, *context.Context, chan<- camtypes.BlobMeta) error) {
+	idx := index.NewMemoryIndex()
+	idxd := indextest.NewIndexDeps(idx)
+	idxd.Fataler = t
+	c, err := idxd.Index.KeepInMemory()
+	if err != nil {
+		t.Fatalf("error slurping index to memory: %v", err)
+	}
+	donec := make(chan struct{})
+	go func() {
+		for i := 0; i < 100; i++ {
+			nth := fmt.Sprintf("%d", i)
+			pn := idxd.NewPlannedPermanode(nth)
+			idxd.SetAttribute(pn, "tag", nth)
+		}
+		donec <- struct{}{}
+	}()
+	go func() {
+		for i := 0; i < 10; i++ {
+			ch := make(chan camtypes.BlobMeta, 10)
+			errc := make(chan error, 1)
+			c.RLock()
+			go func() { errc <- enumFunc(c, context.TODO(), ch) }()
+			for _ = range ch {
+			}
+			err := <-errc
+			c.RUnlock()
+			if err != nil {
+				t.Fatalf("Could not enumerate permanodes: %v", err)
+			}
+		}
+		donec <- struct{}{}
+	}()
+	<-donec
+	<-donec
+}
+
+func TestLazySortedPermanodes(t *testing.T) {
+	idx := index.NewMemoryIndex()
+	idxd := indextest.NewIndexDeps(idx)
+	idxd.Fataler = t
+	c, err := idxd.Index.KeepInMemory()
+	if err != nil {
+		t.Fatalf("error slurping index to memory: %v", err)
+	}
+
+	lsp := c.Exp_LSPByTime(false)
+	if len(lsp) != 0 {
+		t.Fatal("LazySortedPermanodes cache should be empty on startup")
+	}
+
+	pn := idxd.NewPlannedPermanode("one")
+	idxd.SetAttribute(pn, "tag", "one")
+
+	enum := func(reverse bool) {
+		ch := make(chan camtypes.BlobMeta, 10)
+		errc := make(chan error, 1)
+		c.RLock()
+		go func() { errc <- c.EnumeratePermanodesCreatedLocked(context.TODO(), ch, reverse) }()
+		for _ = range ch {
+		}
+		err := <-errc
+		c.RUnlock()
+		if err != nil {
+			t.Fatalf("Could not enumerate permanodes: %v", err)
+		}
+	}
+	enum(false)
+	lsp = c.Exp_LSPByTime(false)
+	if len(lsp) != 1 {
+		t.Fatalf("LazySortedPermanodes after 1st enum: got %v items, wanted 1", len(lsp))
+	}
+	lsp = c.Exp_LSPByTime(true)
+	if len(lsp) != 0 {
+		t.Fatalf("LazySortedPermanodes reversed after 1st enum: got %v items, wanted 0", len(lsp))
+	}
+
+	enum(true)
+	lsp = c.Exp_LSPByTime(false)
+	if len(lsp) != 1 {
+		t.Fatalf("LazySortedPermanodes after 2nd enum: got %v items, wanted 1", len(lsp))
+	}
+	lsp = c.Exp_LSPByTime(true)
+	if len(lsp) != 1 {
+		t.Fatalf("LazySortedPermanodes reversed after 2nd enum: got %v items, wanted 1", len(lsp))
+	}
+
+	pn = idxd.NewPlannedPermanode("two")
+	idxd.SetAttribute(pn, "tag", "two")
+
+	enum(true)
+	lsp = c.Exp_LSPByTime(false)
+	if len(lsp) != 0 {
+		t.Fatalf("LazySortedPermanodes after 2nd permanode: got %v items, wanted 0 because of cache invalidation", len(lsp))
+	}
+	lsp = c.Exp_LSPByTime(true)
+	if len(lsp) != 2 {
+		t.Fatalf("LazySortedPermanodes reversed after 2nd permanode: got %v items, wanted 2", len(lsp))
+	}
+
+	pn = idxd.NewPlannedPermanode("three")
+	idxd.SetAttribute(pn, "tag", "three")
+
+	enum(false)
+	lsp = c.Exp_LSPByTime(true)
+	if len(lsp) != 0 {
+		t.Fatalf("LazySortedPermanodes reversed after 3rd permanode: got %v items, wanted 0 because of cache invalidation", len(lsp))
+	}
+	lsp = c.Exp_LSPByTime(false)
+	if len(lsp) != 3 {
+		t.Fatalf("LazySortedPermanodes after 3rd permanode: got %v items, wanted 3", len(lsp))
+	}
+
+	enum(true)
+	lsp = c.Exp_LSPByTime(false)
+	if len(lsp) != 3 {
+		t.Fatalf("LazySortedPermanodes after 5th enum: got %v items, wanted 3", len(lsp))
+	}
+	lsp = c.Exp_LSPByTime(true)
+	if len(lsp) != 3 {
+		t.Fatalf("LazySortedPermanodes reversed after 5th enum: got %v items, wanted 3", len(lsp))
+	}
+}
diff --git a/pkg/index/export_test.go b/pkg/index/export_test.go
index 2da0c2f35..46ab15b8e 100644
--- a/pkg/index/export_test.go
+++ b/pkg/index/export_test.go
@@ -79,3 +79,30 @@ func (x *Index) Exp_AwaitReindexing(t *testing.T) {
 	}
 	t.Fatal("timeout waiting for readyReindex to drain")
 }
+
+type ExpPnAndTime pnAndTime
+
+// Exp_LSPByTime returns the sorted cache lazySortedPermanodes for
+// permanodesByTime (or the reverse sorted one).
+func (c *Corpus) Exp_LSPByTime(reverse bool) []ExpPnAndTime {
+	if c.permanodesByTime == nil {
+		return nil
+	}
+	var pn []ExpPnAndTime
+	if reverse {
+		if c.permanodesByTime.sortedCacheReversed != nil {
+			for _, v := range c.permanodesByTime.sortedCacheReversed {
+				pn = append(pn, ExpPnAndTime(v))
+			}
+			return pn
+		}
+	} else {
+		if c.permanodesByTime.sortedCache != nil {
+			for _, v := range c.permanodesByTime.sortedCache {
+				pn = append(pn, ExpPnAndTime(v))
+			}
+			return pn
+		}
+	}
+	return nil
+}