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I have 2 drives setup in mdadm mirroring (3 partitions on each drive). Last week the second drive (sdb) dropped out of the array because of some I/O errors I am still investigating. After a server reboot the /proc/mdstat reported the array degraded as expected.

Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
md2 : active raid1 sda3[0]
      1410613056 blocks super 1.0 [2/1] [U_]
      bitmap: 11/11 pages [44KB], 65536KB chunk

md1 : active (auto-read-only) raid1 sda2[0]
      2097088 blocks super 1.0 [2/1] [U_]
      bitmap: 1/1 pages [4KB], 65536KB chunk

md0 : active raid1 sda1[0]
      52426624 blocks super 1.0 [2/1] [U_]
      bitmap: 1/1 pages [4KB], 65536KB chunk

I have re-added the first two partitions to he corresponding arrays because they were small (50Gb, and 2Gb), and left the third out because I didn't want to wait for the sync (1.5Tb sync lasts > 5hrs). A grep of the /var/log/messages for the third partition looks like this:

2014-08-29T07:02:09.168903+02:00 cube kernel: [   11.460362] md: bind<sdb3>
2014-08-29T07:02:09.168908+02:00 cube kernel: [   11.681545] md: bind<sda3>
2014-08-29T07:02:09.168913+02:00 cube kernel: [   11.692178] md: kicking non-fresh sdb3 from array!
2014-08-29T07:02:09.168917+02:00 cube kernel: [   11.692209] md: unbind<sdb3>
2014-08-29T07:02:09.168922+02:00 cube kernel: [   11.700679] md: export_rdev(sdb3)
2014-08-29T07:02:09.168942+02:00 cube kernel: [   11.706311] md/raid1:md2: active with 1 out of 2 mirrors
2014-08-29T07:02:09.168949+02:00 cube kernel: [   11.829345] created bitmap (11 pages) for device md2
2014-08-29T07:02:09.168954+02:00 cube kernel: [   11.877779] md2: bitmap initialized from disk: read 1 pages, set 859 of 21525 bits
2014-08-29T07:02:09.168959+02:00 cube kernel: [   11.915548] md2: detected capacity change from 0 to 1444467769344
2014-08-28T20:17:30.195301+02:00 cube kernel: [   10.713443] RAID1 conf printout:
2014-08-28T20:17:30.195306+02:00 cube kernel: [   10.713459]  --- wd:1 rd:2
2014-08-28T20:17:30.195333+02:00 cube kernel: [   10.713469]  disk 0, wo:0, o:1, dev:sda3
2014-08-28T20:17:30.195342+02:00 cube kernel: [   10.810753]  md2: unknown partition table

Today I powered up the server and mdadm re-added the third partition all by itself. First I thought I did it, but my command history doesn't have this command issued. Is there a way that this is some hidden mdadm option/feature? The grep from todays /var/log/messages is very strange:

2014-08-31T11:31:42.388226+02:00 cube kernel: [   13.240221] md: bind<sda3>
2014-08-31T11:31:42.388234+02:00 cube kernel: [   13.294663] md/raid1:md2: active with 1 out of 2 mirrors
2014-08-31T11:31:42.388238+02:00 cube kernel: [   13.382804] created bitmap (11 pages) for device md2
2014-08-31T11:31:42.388243+02:00 cube kernel: [   13.387596] md2: bitmap initialized from disk: read 1 pages, set 859 of 21525 bits
2014-08-31T11:31:42.388248+02:00 cube kernel: [   13.433121] md2: detected capacity change from 0 to 1444467769344
2014-08-31T11:31:42.388253+02:00 cube kernel: [   13.445851]  md2: unknown partition table
2014-08-31T11:31:42.388258+02:00 cube kernel: [   13.448115] md: bind<sdb3>
2014-08-31T11:31:42.388276+02:00 cube kernel: [   13.448230] RAID1 conf printout:
2014-08-31T11:31:42.388283+02:00 cube kernel: [   13.448237]  --- wd:1 rd:2
2014-08-31T11:31:42.388288+02:00 cube kernel: [   13.448245]  disk 0, wo:0, o:1, dev:sda3
2014-08-31T11:31:42.388292+02:00 cube kernel: [   13.448252]  disk 1, wo:1, o:1, dev:sdb3
2014-08-31T11:31:42.388297+02:00 cube kernel: [   13.448266] RAID1 conf printout:
2014-08-31T11:31:42.388302+02:00 cube kernel: [   13.448271]  --- wd:2 rd:2
2014-08-31T11:31:42.388306+02:00 cube kernel: [   13.448277]  disk 0, wo:0, o:1, dev:sda3
2014-08-31T11:31:42.388374+02:00 cube kernel: [   13.448284]  disk 1, wo:0, o:1, dev:sdb3
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Anything in /etc/mdadm.conf ? –  b13n1u Aug 31 at 12:00
    
Nothing special: DEVICE containers partitions ARRAY /dev/md0 UUID=19810c09:63bbdbbe:aa559d14:e978dab3 ARRAY /dev/md1 UUID=412bd44a:81cd2454:25da078c:481eef1e ARRAY /dev/md2 UUID=442458ed:1efdc068:2a229d4c:604d9582 –  Mladen Marinović Aug 31 at 16:08

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