Good drive got kicked out - does mdadm assemble across suspend/resume like it does across reboots?

Good drive got kicked out - does mdadm assemble across suspend/resume like it does across reboots?

[Ram Ramesh]

I did a minor booboo with my mdadm raid6. I suspended (not shutdown, but 
suspend-to-ram) the machine to install a *new* disk in a hotswap bay for 
a thorough test before usage. This machine also housed a mdadm raid6 
array with 4 disks /dev/sd[cdeg]1 (not /dev/sdf1). When I inserted the 
new disk, I also rearranged the disks in the hotswap cage. I think that 
is a stupid mistake because /dev/sdg1 was also in the cage and now 
became /dev/sdf1 and the new disk got the name /dev/sdg1. I did not 
think much about it, since I thought mdadm would assemble after a resume 
and will find the right disks. I did not bother to check if anything 
happened to /dev/md0.

After checking the new disk (now /dev/sdg1) I added /dev/sdg1 to md0 (as 
spare) without checking /proc/mdstat. I noticed rebuild started to my 
surprise. I was trying to figure out what happened. It looks like resume 
after suspend does not assemble md0 as it does for reboot. So, it looked 
at the new /dev/sdg1 and found to be not part of md0 and degraded the 
array. So when I added it, it thought I am adding a new drive and 
proceeded to rebuild to fix the degraded state. A quick check of 
/dev/sdf1 (which was /dev/sdg1 before my swap) contains everything 
right, but got kicked out of array due to suspend-swap disk-resume issue.

While this is not a disaster, I wonder if my understanding is correct? 
Does this mean mdadm does not scan to assemble as part of resume? More 
importantly, how should I rectify the situation like this? Reboot or 
simply stop md0 and scan in a live system?
Further, how do I use /dev/sdf1 (that was /dev/sdg1 before this 
problem)? zero-superblock and add back to md0?

Sorry, if this was already discussed. If so, just let me know and I will 
search the archive manually as Google did not find it.

Thanks for your help
Ramesh

Re: Good drive got kicked out - does mdadm assemble across suspend/resume like it does across reboots?

[Phillip Susi]

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On 10/12/2014 1:38 PM, Ram Ramesh wrote:
> While this is not a disaster, I wonder if my understanding is
> correct? Does this mean mdadm does not scan to assemble as part of
> resume? More importantly, how should I rectify the situation like
> this? Reboot or simply stop md0 and scan in a live system? Further,
> how do I use /dev/sdf1 (that was /dev/sdg1 before this problem)?
> zero-superblock and add back to md0?

Of course it does not reassemble the array after a suspend -- the
point of suspend/resume is that the system picks up exactly where it
left off.  Since the array is never torn down, it is not built back
up.  In other words, yanking out drives while suspended isn't really
any different than doing it while running -- not a good idea.


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