Re: Help understanding the root cause of a member dropping out of a RAID 1 set.

[John Robinson <john.robinson@anonymous.org.uk>]

On 14/08/2009 14:09, Paweł Brodacki wrote:
> 2009/8/13 John Robinson <john.robinson@anonymous.org.uk>:
> 
>> Can or could md be made or configured to try re-adding a device if this
>> sort of thing happens? After all, a stray cosmic ray or whatever perhaps
>> shouldn't make one lose redundancy if the drive's actually OK?
> 
> I think that from the coding point of view md probably could. The more
> important thing is if it should. The only hard fact is that there was
> an error while accessing the device. md has no way of telling if it
> was just a freak accident, or the drive is unreliable from now on.

Ah well, perhaps we need to give md a way of knowing the difference 
between a transient error (that has been recovered from) and a more 
serious error.

> Therefore it does the one safe thing and says "I won't trust you
> anymore.". If a human being knows better, the said being is free to
> re-add the drive.
> 
> Personally I'd hate having a suspicious drive being auto-added in hope
> it will rebuild and function properly.

I wouldn't want it to be the default behaviour, but I'd like the option 
to configure things that way. I'd want the number of auto-re-adds 
configurable too.

> Because such an option could seem tempting but could and would cause
> loss of reliability I'd expect bad publicity if it was actually added.

But it could cause improvements in reliability too. If the cable on 
drive A is hit by cosmic rays, the drive is taken out of the array, but 
the drive's actually still fine, then drive B fails before the operator 
has re-added drive A, the array goes down when it didn't need to.

What is the operator's most likely response to seeing the SATA bus 
reset? She's going to re-add the drive assuming it was a transient 
error. If we could make this happen automatically, we could close a 
window when the array's more vulnerable. I wouldn't suggest we do it 
silently; it gets logged, notified etc. just like the drive being taken 
out of the array would be.

Cheers,

John.

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