Re: Last working drive in RAID1

[NeilBrown <neilb@suse.de>]

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On Wed, 04 Mar 2015 15:48:57 -0700 Eric Mei <meijia@gmail.com> wrote:

> Hi Neil,
> 
> I see, that does make sense. Thank you.
> 
> But it impose a problem for HA. We have 2 nodes as active-standby pair, 
> if HW on node 1 have problem (e.g. SAS cable get pulled, thus all access 
> to physical drives are gone), we hope the array failover to node 2. But 
> with lingering drive reference, mdadm will report array is still alive 
> thus failover won't happen.
> 
> I guess it depends on what kind of error on the drive. If it's just a 
> media error we should keep it online as much as possible. But if the 
> drive is really bad or physically gone, keeping the stale reference 
> won't help anything. Back to your comparison with single drive /dev/sda, 
> I think MD as an array should do the same as /dev/sda, not the 
> individual drive inside MD, for them we should just let it go. How do 
> you think?

If there were some what that md could be told that the device really was gone
and just just returning errors, then I would be OK with it being marked as
faulty and being removed from the array.

I don't think there is any mechanism in the kernel to allow that.  It would
be easiest to capture a "REMOVE" event via udev, and have udev run "mdadm" to
tell the md array that the device was gone.

Currently there is no way to do that ... I guess we could change raid1 so
that a 'fail' event that came from user-space  would always cause the device
to be marked failed, even when an IO error would not...
To preserve current behaviour, it should require something like "faulty-force"
to be written to the "state" file.   We would need to check that raid1 copes
with having zero working drives - currently it might always assume there is
at least one device.

NeilBrown


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