Fixed the documentation to match reality on darwin.

fuseops/ops.go

@@ -51,9 +51,16 @@ type StatFSOp struct {
 	// of f_frsize. On OS X this is surfaced as statfs::f_bsize, which plays the
 	// same roll.
 	//
+	// It appears as though the original intent of statvfs::f_frsize in the posix
+	// standard was to support a smaller addressable unit than statvfs::f_bsize
+	// (cf. The Linux Programming Interface by Michael Kerrisk,
+	// https://goo.gl/5LZMxQ). Therefore users should probably arrange for this
+	// to be no larger than IoSize.
+	//
 	// On Linux this can be any value, and will be faithfully returned to the
-	// caller of statfs(2) (see the code walk above). On OS X it appears it must
-	// be a power of 2 in the range [2^9, 2^17].
+	// caller of statfs(2) (see the code walk above). On OS X it appears that
+	// only powers of 2 in the range [2^9, 2^17] are preserved, and a value of
+	// zero is treated as 4096.
 	//
 	// This interface does not distinguish between blocks and block fragments.
 	BlockSize uint32
@@ -77,8 +84,9 @@ type StatFSOp struct {
 	// statfs::f_iosize. Both are documented in `man 2 statfs` as "optimal
 	// transfer block size".
 	//
-	// On Linux this can be any value. On OS X it appears it must be a power of 2
-	// in the range [2^9, 2^20].
+	// On Linux this can be any value. On OS X it appears that only powers of 2
+	// in the range [2^12, 2^20] are faithfully preserved, and a value of zero is
+	// treated as 65536.
 	IoSize uint32
 
 	// The total number of inodes in the file system, and how many remain free.

samples/statfs/statfs_darwin_test.go

@@ -170,27 +170,23 @@ func (t *StatFSTest) IoSizes() {
 		fsIoSize       uint32
 		expectedIosize uint32
 	}{
-		0:  {0, 4096},
-		1:  {1, 512},
-		2:  {3, 512},
-		3:  {511, 512},
-		4:  {512, 512},
-		5:  {513, 1024},
-		6:  {1023, 1024},
-		7:  {1024, 1024},
-		8:  {4095, 4096},
-		9:  {1 << 16, 1 << 16},
-		10: {1<<17 - 1, 1 << 17},
-		11: {1 << 17, 1 << 17},
-		12: {1<<17 + 1, 1 << 18},
-		13: {1<<20 - 1, 1 << 20},
-		14: {1 << 20, 1 << 20},
-		15: {1<<20 + 1, 1 << 20},
-		16: {math.MaxInt32 - 1, 1 << 20},
-		17: {math.MaxInt32, 1 << 20},
-		18: {math.MaxInt32 + 1, 512},
-		19: {math.MaxInt32 + 1<<15, 1 << 15},
-		20: {math.MaxUint32, 1 << 20},
+		0:  {0, 65536},
+		1:  {1, 4096},
+		2:  {3, 4096},
+		3:  {4095, 4096},
+		4:  {4096, 4096},
+		5:  {4097, 8192},
+		6:  {8191, 8192},
+		7:  {8192, 8192},
+		8:  {8193, 16384},
+		9:  {1<<20 - 1, 1 << 20},
+		10: {1 << 20, 1 << 20},
+		11: {1<<20 + 1, 1 << 20},
+		12: {math.MaxInt32 - 1, 1 << 20},
+		13: {math.MaxInt32, 1 << 20},
+		14: {math.MaxInt32 + 1, 4096},
+		15: {math.MaxInt32 + 1<<15, 1 << 15},
+		16: {math.MaxUint32, 1 << 20},
 	}
 
 	for i, tc := range testCases {