Re: Weirdness with DDF arrays (mdadm 3.3)

[Martin Wilck <mwilck@arcor.de>]

On 09/16/2013 03:47 PM, Francis Moreau wrote:
[...]

>>>> Wrt loop1[2], I think you interpret the [2] wrongly. It seems to be the
>>>> kernel index of the device somehow. The mdstat parsing code of mdadm
>>>> doesn't look at this number. If you look at
>>>> /sys/class/block/md124/md/dev-loop*/slot, the number should be correct -
>>>> I tried it here.
>>>
>>> Well I think I interpreted the numbers the way it's described here :
>>>
>>> https://raid.wiki.kernel.org/index.php/Mdstat#md_device_line
>>
>> That description is not quite correct. The number in brackets [2] means
>> the index of the disk in the meta data (for DDF, that's the index in the
>> "physical disks" table of the container). That number isn't very
>> meaningful except for the meta data itself.
>>
>> The logical disk index is represented by the "slot" attribute in sysfs.
>>
>> See e.g.
>> http://lxr.missinglinkelectronics.com/linux+*/drivers/md/md.h#L79
>>
>> The number displayed in /proc/mdstat is "desc_nr", while the number that
>> actually matters is "raid_disk".
> 
> Maybe the description in the wiki is correct but there's a bun in the
> kernel which displays the wrong number ?
>
> If "desc_nr" isn't meaningful, I don't see the point to show it in
> /proc/mdstat.

Well, we could propose to change the value displayed by the kernel. The
question is whether the kernel maintainers would allow that, because it
would change the kernel-userspace API. mdadm itself doesn't look at this
number, but other tools might, so there is some small but non-zero risk
of breaking something somewhere.

Let's wait what others have to say.

Martin