I'm lucky, I reproduced it again and I ran the command udevadm monitor > monitor before test.
I'll put it in as an attachment.

  I found there is a not whole message in the last line. 

Xiao

----- Original Message -----

From: "Xiao Ni" <redacted>
To: "NeilBrown" <redacted>
Cc: linux-raid@vger.kernel.org
Sent: Thursday, March 26, 2015 3:36:35 PM
Subject: Re: /dev/md0 can't be created

Hi Neil

   I can reproduce it now. Do you want me to run udevadm monitor before the
   test?
And I checked the /var/log/messages, it show the information:

Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-2>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-3>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-4>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-5>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-6>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-7>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-9>
Oct 20 19:59:59 ibm-z10-25 kernel: md: bind<dm-8>
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-7 operational as
raid disk 5
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-6 operational as
raid disk 4
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-5 operational as
raid disk 3
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-4 operational as
raid disk 2
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-3 operational as
raid disk 1
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: device dm-2 operational as
raid disk 0
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: allocated 0kB
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid:md0: raid level 5 active with 6
out of 7 devices, algorithm 2
Oct 20 19:59:59 ibm-z10-25 kernel: md/raid456: discard support disabled due
to uncertainty.
Oct 20 19:59:59 ibm-z10-25 kernel: Set
raid456.devices_handle_discard_safely=Y to override.
Oct 20 19:59:59 ibm-z10-25 kernel: md0: detected capacity change from 0 to
1881145344
Oct 20 19:59:59 ibm-z10-25 kernel: md: recovery of RAID array md0
Oct 20 19:59:59 ibm-z10-25 kernel: md: minimum _guaranteed_  speed: 1000
KB/sec/disk.
Oct 20 19:59:59 ibm-z10-25 kernel: md: using maximum available idle IO
bandwidth (but not more than 200000 KB/sec) for recovery.
Oct 20 19:59:59 ibm-z10-25 kernel: md: using 128k window, over a total of
306176k.
Oct 20 19:59:59 ibm-z10-25 systemd-udevd: inotify_add_watch(7, /dev/md0, 10)
failed: No such file or directory
  
Xiao

----- Original Message -----

quoted

From: "NeilBrown" <redacted>
To: "Xiao Ni" <redacted>
Cc: linux-raid@vger.kernel.org
Sent: Wednesday, March 25, 2015 2:35:29 PM
Subject: Re: /dev/md0 can't be created

On Wed, 25 Mar 2015 02:15:34 -0400 (EDT) Xiao Ni [off-list ref] wrote:

quoted

Hi all

   I have encountered so many times, the raid device is created
   successfully, but the directory
/dev/md0 can't be created. It can't reproduce 100%.

[root@intel-sugarbay-do-01 create_assemble]# cat /proc/mdstat
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid10 loop7[7](S) loop6[6] loop5[5] loop4[4] loop3[3]
loop2[2] loop1[1] loop0[0]
      1788416 blocks super 1.2 512K chunks 2 near-copies [7/7] [UUUUUUU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

unused devices: <none>
[root@intel-sugarbay-do-01 create_assemble]# ls /dev/md0
ls: cannot access /dev/md0: No such file or directory

    The underline devices are loop devices which are created with big
    file.

    The kernel I used is RHEL7 (3.10.0-234.el7.x86_64.debug, mdadm -
    v3.3.2
    - 21st August 2014)
I'll try to reproduce this with upstream kernel and mdadm. But I think it
shouldn't be the problem about kernel.

    What do you think I should check for this? And which tool is
    responsible for creating the directory? Maybe
I can add some log to it to find the reason.

/dev/md0 is created by udev.
Run
  udevadm monitor

to see the events that udev is processing.  When and ADD event for "md0" is
processed, /dev/md0 should get created.

NeilBrown

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