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Re: SATA hotplug + mdadm raid

On Thu, May 18, 2017 at 11:49:11AM -0500, Sam Smith wrote:
> Hi,
> I recently upgraded my home server to an HP ml30 tower server. It came with
> a 4 drive hotplug SATA cage. I loaded two old unused drives in it and
> installed Debian Stretch on it putting the drives in a software raid1 via
> the debian installer. My plan was once I got the new box up and running that
> I would shut off the old machine and use the drives from that, swapping them
> out one at a time and letting mdadm resync.
> Before putting the new box into "production", I pulled the 2nd drive (sdb)
> to see what would happen (with the machine still running). Of course mdadm
> went into degraded mode with a one disk raid1. I slid the same drive back in
> and it came back as /dev/sdb. Later on, after a few reboots, I pulled the
> 2nd drive and swapped it with a drive from the old machine (it was also
> still up and running with mdadm raid1). However this time the new drive
> didn't come up as sdb, but as /dev/sdc. I set up the partitions the same as
> sda but I didn't join it to the mdadm array yet. I wasn't sure if joining as
> "sdc" and then on reboot having it come up possibly as "sdb" would mess up
> anything so I just rebooted. On reboot it came up as "sdb" and I joined it
> to the array and all was well.
> But I still need to swap the first drive (sda) and I don't really want to
> have to reboot this time. So what can I do to ensure that once I pull the
> old drive and put in the new one that it comes back up as "sda"? Or does
> that even matter? (seems like it would..)

It doesn't matter that much. Use this:

mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1
mdadm --add /dev/md0 /dev/whatever1

then check on progress with cat /proc/mdstat


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