Repair a Faulty Disk in Raid-5

Quick notes

Identify slow disk

# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   2502 MB in  2.00 seconds = 1251.34 MB/sec
 Timing buffered disk reads: 538 MB in  3.01 seconds = 178.94 MB/sec

# hdparm -Tt /dev/sdb

/dev/sdb:
 Timing cached reads:   2490 MB in  2.00 seconds = 1244.86 MB/sec
 Timing buffered disk reads: 536 MB in  3.01 seconds = 178.31 MB/sec

# hdparm -Tt /dev/sdc

/dev/sdc:
 Timing cached reads:   2524 MB in  2.00 seconds = 1262.21 MB/sec
 Timing buffered disk reads: 538 MB in  3.00 seconds = 179.15 MB/sec

# hdparm -Tt /dev/sdd

/dev/sdd:
 Timing cached reads:   2234 MB in  2.00 seconds = 1117.20 MB/sec
 Timing buffered disk reads: read(2097152) returned 929792 bytes

Set disk to Faulty State and Remove it

#  mdadm --manage /dev/md0 --fail /dev/sdd
mdadm: set /dev/sdd faulty in /dev/md0

#  mdadm --manage /dev/md0 --remove  /dev/sdd
mdadm: hot removed /dev/sdd from /dev/md0

Verify Status

# mdadm --verbose --detail /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Feb  6 15:06:34 2014
     Raid Level : raid5
     Array Size : 2929893888 (2794.16 GiB 3000.21 GB)
  Used Dev Size : 976631296 (931.39 GiB 1000.07 GB)
   Raid Devices : 4
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Mon Jul  8 00:51:14 2019
          State : clean, degraded
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 512K

           Name : ServerOne:0  (local to host ServerOne)
           UUID : d635095e:50457059:7e6ccdaf:7da91c9b
         Events : 18122

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       6       8       32        1      active sync   /dev/sdc
       4       0        0        4      removed
       4       8        0        3      active sync   /dev/sda

Format Disk

quick format to identify bad blocks, better solution zeroing the disk

# mkfs.ext4 /dev/sdd 

# dd if=/dev/zero of=/dev/sdd bs=4096 count=4096

Using dd to remove any ext headers

Test disk

# hdparm -Tt /dev/sdd

/dev/sdd:
 Timing cached reads:   2174 MB in  2.00 seconds = 1087.20 MB/sec
 Timing buffered disk reads: 516 MB in  3.00 seconds = 171.94 MB/sec

Add Disk to Raid

# mdadm --add /dev/md0 /dev/sdd
mdadm: added /dev/sdd

# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd[5] sda[4] sdc[6] sdb[0]
      2929893888 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UU_U]
      [>....................]  recovery =  0.0% (44032/976631296) finish=369.5min speed=44032K/sec

unused devices: <none>

Verify Raid

# mdadm --verbose --detail /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Feb  6 15:06:34 2014
     Raid Level : raid5
     Array Size : 2929893888 (2794.16 GiB 3000.21 GB)
  Used Dev Size : 976631296 (931.39 GiB 1000.07 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Mon Jul  8 00:58:38 2019
          State : clean, degraded, recovering
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 512K

 Rebuild Status : 0% complete

           Name : ServerOne:0  (local to host ServerOne)
           UUID : d635095e:50457059:7e6ccdaf:7da91c9b
         Events : 18244

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       6       8       32        1      active sync   /dev/sdc
       5       8       48        2      spare rebuilding   /dev/sdd
       4       8        0        3      active sync   /dev/sda