server: save and compare multiple disks are used (#1474)

When server is run with multiple disks which uses xl interface where
order and count of disks are important, this patch saves such disks
configuration and compares in next run if there is a mismatch.

Fixes #1458

routers.go

@@ -17,7 +17,9 @@
 package main
 
 import (
+	"fmt"
 	"net/http"
+	"reflect"
 
 	router "github.com/gorilla/mux"
 )
@@ -36,6 +38,26 @@ func newObjectLayer(exportPaths ...string) (ObjectLayer, error) {
 
 // configureServer handler returns final handler for the http server.
 func configureServerHandler(srvCmdConfig serverCmdConfig) http.Handler {
+	// FIXME: currently we don't check single exportPath which uses FS layer.
+	if len(srvCmdConfig.exportPaths) > 1 {
+		if isFormatConfigFileExists() {
+			format, err := getFormatXL()
+			if err != nil {
+				fatalIf(err, "Failed to read format.json", nil)
+			}
+
+			if !reflect.DeepEqual(format.Disks, srvCmdConfig.exportPaths) {
+				err = fmt.Errorf("Number of export paths from command-line did not match the backend configuration. Backend is configured with [%s] exports.", format.Disks)
+				fatalIf(err, "", nil)
+			}
+		} else {
+			// First run: save disk configuration
+			if err := saveFormatXL(xlFormat{Version: "1", Disks: srvCmdConfig.exportPaths}); err != nil {
+				fatalIf(err, "Unable to save 'format.json'", nil)
+			}
+		}
+	}
+
 	objAPI, err := newObjectLayer(srvCmdConfig.exportPaths...)
 	fatalIf(err, "Initializing object layer failed.", nil)