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

Tue Oct 3 00:22:39 UTC 2006
rado <rado at rivers-bend.com>

On Mon, 2006-10-02 at 13:10 -0700, Scott Silva wrote:
> 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.
We talking raid 1 Scott. It actually serves a duel purpose. It's part of
a backup scheme plus it's a bootable disk image

> 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.
>