Re: Question: how to identify failing disk in a RAID1

[Justin Piszcz <jpiszcz@lucidpixels.com>]

On Wed, 23 Apr 2008, Maurice Hilarius wrote:

> Hi all.
>
> With much appreciated help from Bell Davidsen and Justin Piszcz I recently 
> dealt with a problem with a RAID1 set, caused by a failing hard disk.
>
> At the end, there is one question remaining, which I think is quite 
> important:
> When one has a RAID5 or RAID6, and a disk starts "acting up" mdadm rapidly 
> kicks out the offending device.
> Some might say "too easily" but that is another thread.
>
> On a RAID1 set, until the failing disk completely "packs it in" it remains 
> part of the RAID.
>
> Why??
>
> Some more background:
> Since the issue was reported and explored I have recreated this on a test 
> machine.
> Installed RAID1 with one known good and one know error prone drive.
> Easy to do as the error drive has a thermal issue.
> Keep it cold, no problems, but after 30 minutes use in a +25C room it start 
> to generate data errors.
> I reproduced exactly the problem I saw before:
> Data errors occur, the other drive in the RAID1 set gets "infected" with the 
> bad data, and the file system will get corrupted.
> On BOTH drives.
>
> This is highly reproducible.
>
> In summary:
> 1) RAID1 lacks significant protection from the effects of a data error 
> condition on a failing drive
> 2) I recommend anyone using madadm refrain from using RAID1 until this issue 
> is addressed and resolved.
>
> Thanks again.
I can confirm this, until you actually REBOOT the host with RAID1 only 
then will it kick it out.  Whereas with RAID5, I experienced the same 
thing, it kicks it out right away, would need to wait for the 
linux-raid/developers to answer this one.

Justin.