Add large bucket support for erasure coded backend (#5160)

This PR implements an object layer which
combines input erasure sets of XL layers
into a unified namespace.

This object layer extends the existing
erasure coded implementation, it is assumed
in this design that providing > 16 disks is
a static configuration as well i.e if you started
the setup with 32 disks with 4 sets 8 disks per
pack then you would need to provide 4 sets always.

Some design details and restrictions:

- Objects are distributed using consistent ordering
  to a unique erasure coded layer.
- Each pack has its own dsync so locks are synchronized
  properly at pack (erasure layer).
- Each pack still has a maximum of 16 disks
  requirement, you can start with multiple
  such sets statically.
- Static sets set of disks and cannot be
  changed, there is no elastic expansion allowed.
- Static sets set of disks and cannot be
  changed, there is no elastic removal allowed.
- ListObjects() across sets can be noticeably
  slower since List happens on all servers,
  and is merged at this sets layer.

Fixes #5465
Fixes #5464
Fixes #5461
Fixes #5460
Fixes #5459
Fixes #5458
Fixes #5460
Fixes #5488
Fixes #5489
Fixes #5497
Fixes #5496

cmd/admin-handlers_test.go

@@ -253,10 +253,22 @@ func (atb *adminXLTestBed) CleanupHealTestData(t *testing.T) {
 // initTestObjLayer - Helper function to initialize an XL-based object
 // layer and set globalObjectAPI.
 func initTestXLObjLayer() (ObjectLayer, []string, error) {
-	objLayer, xlDirs, xlErr := prepareXL16()
-	if xlErr != nil {
-		return nil, nil, xlErr
+	xlDirs, err := getRandomDisks(16)
+	if err != nil {
+		return nil, nil, err
 	}
+	endpoints := mustGetNewEndpointList(xlDirs...)
+	format, err := waitForFormatXL(true, endpoints, 1, 16)
+	if err != nil {
+		removeRoots(xlDirs)
+		return nil, nil, err
+	}
+
+	objLayer, err := newXLSets(endpoints, format, 1, 16)
+	if err != nil {
+		return nil, nil, err
+	}
+
 	// Make objLayer available to all internal services via globalObjectAPI.
 	globalObjLayerMutex.Lock()
 	globalObjectAPI = objLayer