[CentOS] more software raid questions

Wed Oct 20 02:46:45 UTC 2010
Nataraj <incoming-centos at rjl.com>

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.
> 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 the drive are out of date and then you only resync pages which need 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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mdadm /dev/mdN --assemble --force
could also be useful, though I would be careful here. 
To use this, you would have to stop all of the arrays and then 
reassemble.  You could also specify the specific drives.
If you don't have a backup, you might want to backup the single drives 
that are properly mounted from md0 and md1.  Data loss is always a 
possibility with these type of manipulations, though I have successfully 
recovered from things like this without losing any data.  In fact I pull 
drives out of a raid array and add new drives in daily to sync them and 
send the second drive off site as a backup.