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/posix.go

@@ -43,6 +43,7 @@ type posix struct {
 	ioErrCount int32 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
 	diskPath   string
 	pool       sync.Pool
+	connected  bool
 }
 
 // checkPathLength - returns error if given path name length more than 255
@@ -137,6 +138,8 @@ func newPosix(path string) (StorageAPI, error) {
 		return nil, err
 	}
 
+	st.connected = true
+
 	// Success.
 	return st, nil
 }
@@ -218,14 +221,13 @@ func (s *posix) String() string {
 	return s.diskPath
 }
 
-// Init - this is a dummy call.
-func (s *posix) Init() error {
+func (s *posix) Close() error {
+	s.connected = false
 	return nil
 }
 
-// Close - this is a dummy call.
-func (s *posix) Close() error {
-	return nil
+func (s *posix) IsOnline() bool {
+	return s.connected
 }
 
 // DiskInfo provides current information about disk space usage,
@@ -249,6 +251,9 @@ func (s *posix) getVolDir(volume string) (string, error) {
 // checkDiskFound - validates if disk is available,
 // returns errDiskNotFound if not found.
 func (s *posix) checkDiskFound() (err error) {
+	if !s.IsOnline() {
+		return errDiskNotFound
+	}
 	_, err = os.Stat((s.diskPath))
 	if err != nil {
 		if os.IsNotExist(err) {