[CentOS] Re: Rebuilding Raid 1

Scott Silva ssilva at sgvwater.com
Wed May 3 16:09:03 UTC 2006

Les Mikesell spake the following on 5/1/2006 9:32 PM:
> On Tue, 2006-05-02 at 00:13, Mace Eliason wrote:
>> I think the reason for the partitions not being on the same drive is 
>> because the old drive is the bootable drive and I changed the scsi ids 
>> of the drives to get it to work.   I think it is using sba for the boot 
>> but centos is on sbb. 
> It doesn't really boot from raid.  It loads the kernel from
> the underlying partition on the boot drive, then the kernel
> detects the raid devices and decides how to activate them,
> then it mounts them according to /etc/fstab.  You should be
> able to review the detection process with 'dmesg |less'
> if you are interested.
>> I just need to make sure that the sync happens so that the info on sbb 
>> is what is synced.  The info on sba is 1 month old this I have confirmed 
>> by mounting sba3 and looking at the dates of the last emails received on 
>> the server  I couldn't mount sdb3 because it is the active partition.
> Be careful with your spelling... I assume you mean /dev/sda3 is the
> old one.  
>> So from what your saying below running mdadm as you have shown will it 
>> copy the good info on sdb to sda?
> Again, be careful.  It will copy the currently active partition
> to the one you are adding - so if you see the current version
> now, that's what will be mirrored.  In the case of /dev/sdb3 it
> will go from sdb to sda.   But /dev/sda1 is active now and will
> be copied to /dev/sdb1.
>> I have to have this server ready for 7am or we will loose this 
>> contract.  I can't believe I have spend all day on this.
> This is the sort of thing you should practice on a test
> box so you are prepared to handle an actual problem in a
> reasonable amount of time.
>> If I copy over the wrong info they will lose 1 months worth of emails.
> You can only copy from the active one to the one you
> add.  And by the way, having backups can be a good thing
> too...
You might want to download the free Vmware server software and play with
senarios like this. You can create installs with 2 drives in raid 1, and kill
a drive and play with rebuilding. All virtual, so no damage to working
systems. This is how I learned some of the nuances of linux software raid.


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