Fsync is slow on mac os

[rjl493456442]

According to issue golang/go#43342, file.Sync operation is extremely slow.

After investigating it a bit further, I realized that for file.Sync on mac os X, it's actually a fcntl(F_FULLFSYNC) because OS X and some versions of ext3 have an fsync that doesn't really flush to disk. OS X requires fcntl(F_FULLFSYNC) to flush to disk

Apparently, F_FULLFSYNC is a lot slower than a normal file.Sync on the other platforms.

func TestFooBar(t *testing.T) {
	filename := "go.txt"
	os.Remove(filename)
	fp, err := os.OpenFile(filename, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
	if err != nil {
		panic(err)
	}
	count := 1000
	for i := 0; i < count; i++ {
		var err error
		s := fmt.Sprintf("%010d\n", i)
		_, err = fp.WriteString(s)
		if err != nil {
			panic(err)
		}
		err = fp.Sync()
		if err != nil {
			panic(err)
		}
	}
	fp.Close()
}

1000 file sync takes 18s on my laptop (Apple M1 Pro)

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Inside of Geth, we use freezer as the chain data backend and state diff backend if path scheme is used, which always do a file.Sync after applying writes. Namely it can cause performance degradation on Mac OS X.

If the node has finished the initial state sync, then capturing up the progress made in the network should be sufficient even with slow FSYNC, but might be a disaster if people want to do a full-sync on Mac OS X.