fuse: change to direct io mode

before and after:

chrislu$ time dd if=/dev/random of=/Users/chrislu/tmp/mm/testfile bs=131072 count=8192
8192+0 records in
8192+0 records out
1073741824 bytes transferred in 4.534068 secs (236816430 bytes/sec)
dd if=/dev/random of=/Users/chrislu/tmp/mm/testfile bs=131072 count=8192  0.01s user 3.86s system 84% cpu 4.561 total
chrislu$ time dd if=/dev/random of=/Users/chrislu/tmp/mm/testfile bs=131072 count=8192
8192+0 records in
8192+0 records out
1073741824 bytes transferred in 3.824072 secs (280784948 bytes/sec)
dd if=/dev/random of=/Users/chrislu/tmp/mm/testfile bs=131072 count=8192  0.01s user 3.22s system 83% cpu 3.857 total

weed/command/mount_std.go

@@ -175,7 +175,7 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		fuse.DefaultPermissions(),
 		fuse.MaxReadahead(1024 * 128),
 		fuse.AsyncRead(),
-		fuse.WritebackCache(),
+		// fuse.WritebackCache(),
 		fuse.MaxBackground(128),
 		fuse.CongestionThreshold(128),
 	}

weed/filesys/file.go

@@ -96,6 +96,7 @@ func (file *File) Getxattr(ctx context.Context, req *fuse.GetxattrRequest, resp
 func (file *File) Open(ctx context.Context, req *fuse.OpenRequest, resp *fuse.OpenResponse) (fs.Handle, error) {
 
 	glog.V(4).Infof("file %v open %+v", file.fullpath(), req)
+	resp.Flags |= fuse.OpenDirectIO
 
 	handle := file.wfs.AcquireHandle(file, req.Uid, req.Gid)