mdadm disk replacement keeps failing topic

Hi,

I have a Raid 5 setup thats been running for years. About 3 time I had a disk fail, and replaced it with a new one. I'm no Linux expert so I have been working mainly through webmin doing all this.

2 months ago I had another disk fail, and I replaced it with a used disk I had laying around (according to sticker its almost 6 years old). At the same time I decided to reinstall the OS. I'm now running Ubuntu Linux 14.10.

A week ago I noticed degraded performance so I checked s.m.a.r.t status, and the the last added disk had 394 errors. I decided to buy a new one and swapped it in yesterday. This process went differently than usual. After boot madam didn't recover the raid. I got it back up by:

Code:

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sudo mdadm --stop /dev/md127

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and

Code:

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mdadm --assemble --force /dev/md127 /dev/sda2 /dev/sdb2 /dev/sdc2 /dev/sdd2

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The raid came up and started to rebuild itself. Speed was ok but after some hours someting happened.

Code:

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henrik@ocean:~$ sudo mdadm --detail /dev/md127[sudo] password for henrik:
/dev/md127:
        Version : 0.90
  Creation Time : Sun Jan 16 16:54:05 2011
    Raid Level : raid5
    Array Size : 5766792000 (5499.64 GiB 5905.20 GB)
  Used Dev Size : 1922264000 (1833.21 GiB 1968.40 GB)
  Raid Devices : 4
  Total Devices : 4
Preferred Minor : 127
    Persistence : Superblock is persistent


    Update Time : Sun Feb  8 07:58:01 2015
          State : clean, FAILED
Active Devices : 2
Working Devices : 3
Failed Devices : 1
  Spare Devices : 1


        Layout : left-symmetric
    Chunk Size : 64K


          UUID : 6b4d814c:5c337c42:e68158d3:bcc31e01
        Events : 0.371442


    Number  Major  Minor  RaidDevice State
      0      0        0        0      removed
      2      0        0        2      removed
      2      8      34        2      active sync  /dev/sdc2
      3      8      50        3      active sync  /dev/sdd2


      4      8        2        -      faulty  /dev/sda2
      5      8      18        -      spare  /dev/sdb2
henrik@ocean:~$ sudo mdadm --detail /dev/md127

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I rebooted and the raid didn't come up. I assembled it again and the same thing happened after some hours.

The disk I replaced is /dev/sdb. Why is it set as spare? Is it working as part of the raid?

Should I replace /dev/sda now? It seems to have a fine s.m.a.r.t report.

The raid is up and I can access the data. But this situation is not stable. Any advice is appreciated.

/Henrik