On Wed, 28 Jan 2015 11:59:30 -0500 John Drescher <drescherjm@gmail.com> wrote:

> On Wed, Jan 28, 2015 at 1:44 AM, lilofile <lilofile@aliyun.com> wrote:
> > kernel  linux 3.10, centos
> > when I use 5 disk to create raid5,after sync complete, I unplug a disk,and wait a moment, then replug the same disk.
> >
> > the original 4 disk are still a raid5 which is degraded.
> > I find the single disk  become a new raid5,but inactive,what the reason?
> 
> stop whatever new array that shows up then use
> 
> mdadm --manage /dev/mymddevice --re-add /dev/disk5
> 

Also, you probably need a newer version of mdadm. Creating a new inactive
raid5 was a bug.

If you have udev configured to run "mdadm -I" on newly added devices (which I
think most distros do), then adding
  POLICY action=re-add

to /etc/mdadm.conf should  cause the device to automatically be added back to
the array when you plug it in.

NeilBrown