Re: /sys/block/md126 still exists even after stopping the array

[Francis Moreau <francis.moro@gmail.com>]

Hi Neil,

On 09/30/2014 09:43 AM, Francis Moreau wrote:
> Hi Neil,
> 
> On 09/29/2014 11:56 PM, NeilBrown wrote:
>> On Mon, 29 Sep 2014 10:45:17 +0200 Francis Moreau <francis.moro@gmail.com>
>> wrote:
>>
>>>> So what were pids 930 and 459?
>>>> One was presumably the "mdadm -Ss"  - probably 930.
>>>> Is 459 the "mdadm --monitor" ??  That might be useful hint.
>>>>
>>>
>>> yes.
>>>
>>> [456] is:  /sbin/mdadm --monitor --scan --daemonise --syslog
>>> --pid-file=/run/mdadm/mdadm.pid
>>>
>>> and [930] is 'mdamd -Ss'.
>>
>> Good.  Please try the patch below.
>>
> 
> After applying your patch, this is what I'm getting in syslog:
> 
> Sep 30 03:40:07 localhost kernel: md_open(): md125 opened by mdadm [970]
> Sep 30 03:40:07 localhost kernel: md_release(): md125 released by mdadm
> [970]
> Sep 30 03:40:07 localhost kernel: md_open(): md125 opened by mdadm [972]
> Sep 30 03:40:07 localhost kernel: md_open(): md125 opened by mdadm [970]
> Sep 30 03:40:07 localhost kernel: md_release(): md125 released by mdadm
> [972]
> Sep 30 03:40:07 localhost kernel: md_open(): md125 opened by
> systemd-udevd [971]
> Sep 30 03:40:07 localhost systemd[1]: Cannot add dependency job for unit
> mdmonitor-takeover.service, ignoring: Invalid argument
> Sep 30 03:40:07 localhost systemd[1]: Started Software RAID monitoring
> and management.
> Sep 30 03:40:07 localhost kernel: md_release(): md125 released by
> systemd-udevd [971]
> Sep 30 03:40:08 localhost mdadm[466]: DeviceDisappeared event detected
> on md device /dev/md125
> Sep 30 03:40:08 localhost mdadm[466]: DeviceDisappeared event detected
> on md device /dev/md126
> Sep 30 03:40:08 localhost mdadm[466]: DeviceDisappeared event detected
> on md device /dev/md127
> Sep 30 03:40:08 localhost kernel: md125: detected capacity change from
> 1863254016 to 0
> Sep 30 03:40:08 localhost kernel: md: md125 stopped.
> Sep 30 03:40:08 localhost kernel: md: unbind<vdc3>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdc3)
> Sep 30 03:40:08 localhost kernel: md: unbind<vdb3>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdb3)
> Sep 30 03:40:08 localhost kernel: md_release(): md125 released by mdadm
> [970]
> Sep 30 03:40:08 localhost kernel: md_open(): md127 opened by mdadm [466]
> Sep 30 03:40:08 localhost kernel: md_release(): md127 released by mdadm
> [466]
> Sep 30 03:40:08 localhost kernel: md_open(): md126 opened by mdadm [466]
> Sep 30 03:40:08 localhost kernel: md_release(): md126 released by mdadm
> [466]
> Sep 30 03:40:08 localhost kernel: md_open(): md126 opened by mdadm [970]
> Sep 30 03:40:08 localhost kernel: md_release(): md126 released by mdadm
> [970]
> Sep 30 03:40:08 localhost kernel: md_open(): md126 opened by mdadm [970]
> Sep 30 03:40:08 localhost kernel: md126: detected capacity change from
> 67043328 to 0
> Sep 30 03:40:08 localhost kernel: md: md126 stopped.
> Sep 30 03:40:08 localhost kernel: md: unbind<vdc1>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdc1)
> Sep 30 03:40:08 localhost kernel: md: unbind<vdb1>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdb1)
> Sep 30 03:40:08 localhost kernel: md_open(): md127 opened by mdadm [466]
> Sep 30 03:40:08 localhost kernel: md_release(): md127 released by mdadm
> [466]
> Sep 30 03:40:08 localhost kernel: md_release(): md126 released by mdadm
> [970]
> Sep 30 03:40:08 localhost kernel: md_open(): md127 opened by mdadm [970]
> Sep 30 03:40:08 localhost kernel: md_release(): md127 released by mdadm
> [970]
> Sep 30 03:40:08 localhost kernel: md_open(): md127 opened by mdadm [970]
> Sep 30 03:40:08 localhost kernel: md127: detected capacity change from
> 214564864 to 0
> Sep 30 03:40:08 localhost kernel: md: md127 stopped.
> Sep 30 03:40:08 localhost kernel: md: unbind<vdc2>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdc2)
> Sep 30 03:40:08 localhost kernel: md: unbind<vdb2>
> Sep 30 03:40:08 localhost kernel: md: export_rdev(vdb2)
> Sep 30 03:40:08 localhost kernel: md_release(): md127 released by mdadm
> [970]
> 
> The ghost device is no more present so your patch seems to have fixed my
> issue. But I must admit I don't really understand what's going on :-/
> 

Since those 'ghost' devices are expected from the MD implementation
point of view, I'm wondering how am I supposed to detect them or maybe
how an application is supposed to recognized online arrays.

My application uses udev to detect et to get information about new
devices. I don't think the information exported by udev is enough to
figure this out. Also please note that since I rely on udev, I can't
really read information on /sys since this information may be out of
sync with the one returned by udev.

Thanks.