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

Les Mikesell lesmikesell at gmail.com
Mon Oct 2 22:21:25 UTC 2006

On Mon, 2006-10-02 at 13:10 -0700, Scott Silva wrote:

> >> 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.

> 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.

It probably wasn't clear from that conversation but it was about
RAID1.  The number of mirrors isn't limited to 2, so you can
create a 3 drive array with one missing.  This gives you 2 full-time
mirrors plus the ability to add an external drive periodically,
let the sync complete, then remove and rotate it offsite.  It is
best to unmount it before the fail/remove step, but it can run
during the sync and you don't have to shut down.

  Les Mikesell
   lesmikesell at gmail.com

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