[CentOS] more software raid questions

Thu Oct 21 15:59:13 UTC 2010
Nataraj <incoming-centos at rjl.com>

fred smith wrote:
> On Tue, Oct 19, 2010 at 07:34:19PM -0700, Nataraj wrote:
>> I've seen this kind of thing happen when the autodetection stuff 
>> misbehaves. I'm not sure why it does this or how to prevent it. Anyway, 
>> to recover, I would use something like:
>> mdadm --stop /dev/md125
>> mdadm --stop /dev/md126
>> If for some reason the above commands fail, check and make sure it has 
>> not automounted the file systems from md125 and md126. Hopefully this 
>> won't happen.
>> Then use:
>> mdadm /dev/md0 -a /dev/sdXX
>> To add back the drive which belongs in md0, and similar for md1. In 
>> general, it won't let you add the wrong drive, but if you want to check use:
>> mdadm --examine /dev/sda1 | grep UUID
>> and so forth for all your drives and find the ones with the same UUID.
> Well, I've already tried to use --fail and --remove on md125 and md126
> but I'm told the members are still active.
> mdadm /dev/md126 --fail /dev/sdb1 --remove /dev/sdb1
> mdadm /dev/md125 --fail /dev/sdb2 --remove /dev/sdb2
You want to use --stop for the md125 and md126. Those are the raid 
devices that are not correct. Once they are stopped, you can take the 
drives from them and return them to md0 and md1 where they belong.

You will need to add the correct drive that was originally paired in 
each raid set, but as I mentioned, it won't let you add the wrong 
drives, so just try adding sdb1 to md0, then if it doesn't work, add it 
to sdb1. You can't fail out drives from arrays that only have one drive.

> 	mdadm /dev/md126 --fail /dev/sdb1 --remove /dev/sdb1
> 	mdadm: set /dev/sdb1 faulty in /dev/md126
> 	mdadm: hot remove failed for /dev/sdb1: Device or resource busy
> with the intention of then re-adding them to md0 and md1.
> so I tried:
> mdadm /dev/md0 --fail /dev/sda1 --remove /dev/sda1
> and got a similar message. 
> at which point I knew I was in over my head.
>> When I create my Raid arrays, I always use the option --bitmap=internal. 
>> With this option set, a bitmap is used to keep track of which pages on 
>> the drive are out of date and then you only resync pages which need 
>> updating instead of recopying the whole drive when this happens. In the 
>> past I once added a bitmap to an existing raid1 array using something 
>> like this. This may not be the exact command, but I know it can be done:
>> mdadm /dev/mdN --bitmap=internal
>> Adding the bitmap is very worthwhile and saves time and risk of data 
>> loss by not having to recopy the whole partition.
>> Nataraj
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