Re: MD Remnants After --stop

[NeilBrown <neilb@suse.com>]

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On Sat, Nov 19 2016, Marc Smith wrote:

> On Mon, Nov 7, 2016 at 12:44 AM, NeilBrown <neilb@suse.com> wrote:
>> On Sat, Nov 05 2016, Marc Smith wrote:
>>
>>> Hi,
>>>
>>> It may be that I've never noticed this before, so maybe its not a
>>> problem... after using '--stop' to deactivate/stop an MD array, there
>>> are remnants of it lingering, namely an entry in /sys/block (eg,
>>> /sys/block/md127) and the device node in /dev remains (eg,
>>> /dev/md127).
>>>
>>> Is this normal? Like I said, it probably is, and I've just never
>>> noticed it before. I assume its not going to hurt anything, but is
>>> there a way to clean it up, without rebooting? Obviously I could
>>> remove the /dev entry, but what about /sys/block?
>>>
>>
>> You can remove them both by running
>>   mdadm -S /dev/md127
>>
>> but they'll probably just reappear again.
>>
>> This seems to be an on-going battle between md and udev.  I've "fixed"
>> it at least once, but it keeps coming back.
>>
>> When md removes the md127 device, a message is sent to udev.
>> As part of its response to this message, udev tries to open /dev/md127.
>> Because of the rather unusual way that md devices are created (it made
>> sense nearly 20 years ago when it was designed), opening /dev/md127
>> causes md to create device md127 again.
>>
>> You could
>>   mv /dev/md127 /dev/md127X
>>   mdadm -S /dev/md127X
>>   rm /dev/md127X
>> that stop udev from opening /dev/md127.  It seems to work reliably.
>>
>> md used to generate a CHANGE event before the REMOVE event, and only the
>> CHANGE event caused udev to open the device file.  I removed that and
>> the problem went away.  Apparently some change has happened to udev and
>> now it opens the file in response to REMOVE as well.
>
> I used "udevadm monitor -pku" to watch the events when running "mdadm
> --stop /dev/md127" and this is what I see:
>
> --snip--
> KERNEL[163074.119778] change   /devices/virtual/block/md127 (block)
> ACTION=change
> DEVNAME=/dev/md127
> DEVPATH=/devices/virtual/block/md127
> DEVTYPE=disk
> MAJOR=9
> MINOR=127
> SEQNUM=3701
> SUBSYSTEM=block
>
> UDEV  [163074.121569] change   /devices/virtual/block/md127 (block)
> ACTION=change
> DEVNAME=/dev/md127
> DEVPATH=/devices/virtual/block/md127
> DEVTYPE=disk
> MAJOR=9
> MINOR=127
> SEQNUM=3701
> SUBSYSTEM=block
> SYSTEMD_READY=0
> USEC_INITIALIZED=370470
> --snip--
>
> I don't see any 'remove' event generated. I should mention if I hadn't
> already that I'm testing md-cluster (--bitmap=clustered), and
> currently using Linux 4.9-rc3.

What version of mdadm are you using?
You need one which contains
Commit: 229e66cb9689 ("Manage.c: Only issue change events for kernels older than 2.6.28")

which hasn't made it into a release yet.  But if you are playing with
md-cluster, I would guess you are using the latest from git...

NeilBrown

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