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I just discovered that my software RAID, which I set up as a RAID-1, is now showing up as a degraded array with a spare. I dropped and re-added the disk that it thinks is "spare", but it's still a spare. How do I make that spare into the mirror?

xen64:/# mdadm --detail /dev/md0
        Version : 00.90
  Creation Time : Mon May  3 20:56:55 2010
     Raid Level : raid1
     Array Size : 973828096 (928.71 GiB 997.20 GB)
  Used Dev Size : 973828096 (928.71 GiB 997.20 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Sun Nov  7 08:18:50 2010
          State : active, degraded, recovering
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

 Rebuild Status : 3% complete

           UUID : 796cb574:f1263d29:de01a688:bcce49e1
         Events : 0.1279509

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       2       8       19        1      spare rebuilding   /dev/sdb3
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up vote 7 down vote accepted

you raid is already rebuilding. "Rebuild Status : 3% complete"

the spare disk is marked "spare rebuilding", so when the rebuild has completed you disk will be marked "active sync"

nothing to do here

by the way, you can already start using the raid, even if it is degraded or still recovering, but heavy disk usage will ofc slow down the recovery process

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