AIX nfs 安装点消失（甚至从父目录中也看不到），直到 umount/mount

Question

我经历过 NFS“聚会”和相关的事情。但今天我看到了一些不同的东西，我很困惑：挂载目录本身不可见（/foo，从 / 不可见），直到我再次卸载/挂载“/foo”（NFS 共享）。这是在 AIX 6.1 上

## grep -p '^/foo' /etc/filesystems
/foo:
        dev             = "/foo_for_client"
        vfs             = nfs
        nodename        = fooserver
        mount           = true
        options         = rw,bg,hard,intr,sec=sys
        account         = false

#df
 (...)
fooserver:/foo_for_client          -         -    -         -     -  /foo
 # shows that "fooserver:foo_for_client" is mounted on /foo mount-point
 # BUT each field instead of showing size, %, etc, shows "-" instead
 (...)

## cd /
## ls -ld /foo
ls: 0653-341 The file /foo does not exist.
  # ??    
## ls -l /
  (...)
fmm
fnn   
  #and here, NO 'foo' at all. No mount-point (nor mounted) . 
  #And there is also no "stalling" of the display while it fetches '/foo' infos
  # (ie, the rest of the '/' dir content appears quickly)
  # ??? this '/foo' mount-dir seems to be somehow "hidden" (NFS pb?) (see it reappear below)
fpp
fqq
  (...)

## umount /foo
 # no errors.

## cd /
## ls -ld /foo
drwxrwxrwx    2 user    user             256 Jan 01 1970  /foo
 # ?? the mount-point is there and does exist? (even though its date is suspicious)
 # why was it 'hidden' from '/' standpoint while the nfs was mounted?

## mount /foo
 # no errors, and everything is ok again...

#df
 (...)
fooserver:/foo_for_client  220200960  69811632 69%   1122    1% /foo
 # Now each field appears as well
 (...)

我理解为“/foo/.”一旦挂载了 nfs 文件系统，dir 就会取代“/foo”挂载目录。当 NFS 出现问题时，它可能会导致“内部”内容不可用。

但我无法理解的是，如何从父目录中也看不到这个“/foo”？ （从“/”，它应该知道有一个“/foo”子目录条目，然后通过 ls 显示它（当安装 NFS 时使用 NFS 信息，或者当安装 NFS 时使用“/”分区信息）。这里：我根本看不到“/foo”（也看不到挂载目录，也看不到挂载的目录）。

我猜这在某种程度上是由于当“ls”查询有关“/foo”目录的信息时 NFS 未能返回信息？但在这种情况下，为什么“/”中的“ls -l”会默默失败，并且该子目录没有任何超时？ （我忘了“束缚”我的 ls -l 尝试......）