[CentOS] Re: adding a usb drive to an existing raid1 set

Les Mikesell spake the following on 9/29/2006 11:48 AM:
> On Fri, 2006-09-29 at 13:23 -0500, rado wrote:
>> so close yet so far or maybe that's just the way it is, I am dumb on it
>> but gettin smarter.
>>
>> the 2 ide drives diff channels of course hda and hdc each w/3 partitions
>> hda1 2 3  hdc1 2 3.
>>
>> I format hot add sda1 2 3 
>> cat /proc/mdstat it's all there
> 
> Unless you created it with 3 members, the 3rd is added as a spare.
> 
>> ...# mdadm /dev/md0 1 2 -f /dev/hdc1 2 3 (fail or fault hdc
>>
>> md automatically picks up sda from spare status and writes to it.
>>
>> it finishes mind you hdc is still under fail status and I reboot.
>>
>> when the sys comes back up, it does not pick up sda anymore but instead
>> just runs -U on all 3 partitions 
> 
> You can add /dev/hdc1 back if it didn't resync automatically.
> 
>> I was under the impression that sda would be there either I missed a
>> step somewhere or something strange going on
> 
> I don't think usb/firewire is detected early enough to be included
> in the raid assembly at bootup.  That's why I set mine up to use
> 2 internal drives all the time but only periodically sync to the
> external drives that are then rotated offsite.  And if you want to
> be sure everything is clean on the copy, you should stop any
> processes running on the mounted partition, and unmount the mount
> point momentarily while you fail the disk.  If you don't, the
> contents would be the same as if the machine had crashed - probably
> still usable, but it could have problems.
> 
Maybe I am too far down the message tree to get this, but I don't see how
taking one drive from a raid 5 array off site does any good. One drive on a
mirror gives you a backup. One drive in a raid5 just gives you a used hard
drive with not much possibility to recover anything.
But feel free to slap some sense into me if I am too far off.
 I could see if you had a raid 5 array, and then mirrored that to a raid 1
with the usb drive opposite the "entire" raid5 array you would have a viable
backup. But you could just as easily mount the drive and rsync to it to get
the same effect.

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