RE: What to do about Offline_Uncorrectable and Pending_Sector in RAID1

["Peter Sangas" <pete@wnsdev.com>]

Hi Wol,


-----Original Message-----
From: Wols Lists [mailto:antlists@youngman.org.uk] 
Sent: Monday, November 14, 2016 7:58 AM
To: Bruce Merry
Cc: linux-raid@vger.kernel.org
Subject: Re: What to do about Offline_Uncorrectable and Pending_Sector in RAID1

On 14/11/16 15:52, Bruce Merry wrote:
> On 13 November 2016 at 23:06, Wols Lists <antlists@youngman.org.uk> wrote:
>> > Sounds like that drive could need replacing. I'd get a new drive 
>> > and do that as soon as possible - use the --replace option of mdadm 
>> > - don't fail the old drive and add the new.
> Would you mind explaining why I should use --replace instead of taking 
> out the suspect drive? I guess I lose redundancy for any writes that 
> occur while the rebuild is happening, but I'd plan to do this with the 
> filesystem unmounted so there wouldn't be any writes.

>Because a replace will copy from the old drive to the new, recovering any failures from the rest of the array. A fail-and-add will have to rebuild the entire new array >from what's left of the old, stressing the old array much more.

>Okay, in your case, it probably won't make an awful lot of difference, but it does make you vulnerable to problems on the "good" drive. To alter your wording >slightly, you lose redundancy for writes AND READS that occur while the array is rebuilding. It's just good practice (and I point it out because --replace is new and >not well known at present).

>Cheers,
>Wol

With respect to the --replace switch and "replacing a failed drive" documented on the wiki here:
https://raid.wiki.kernel.org/index.php/Replacing_a_failed_drive  Can you clear a few things up for me ?

1. If I just want to replace a working drive in a RAID1 and the array is still redundant I can 
issue the following command as in your example:

mdadm /dev/mdN [--fail /dev/sdx1] --remove /dev/sdx1 --add /dev/sdy1

which fails and removes sdx1 and replaces it with sdy1.

Question1. How is this different from first doing a fail/remove on sdx1, physically replacing sdx1 with sdy1 and doing an add on sdy1?


2. If one of the drives as an error in a RAID1 and gets kicked out of the array and the array loses redundancy the wiki has the following example:

mdmad /dev/mdN --re-add /dev/sdX1
mdadm /dev/mdN --add /dev/sdY1 --replace /dev/sdX1 --with /dev/sdY1

Question2.   Is this point here to first try and re-add sdX1 with the "--re-add" (first line above) and if that fails do a replace (second line above)?


Thanks,
Peter


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