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Linux: key points of raid

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RAID stands for “Redundant Array of Independent Disks”.

The RAID levels

Raid 0: stripe mode,no redundancy in this level,not safe for storage.
Raid 1: mirror mode,having redundancy and good at backup.
Raid 1+0:it is the combination of RAID-1 and RAID-0.
Raid 5: at least 3 disks, can stand max 1 disk broken
Raid 6:at least 4 disks, can stand max 2 disks broken

config file

/etc/mdadm.conf
/proc/mdstat

Create

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[root@client ~]# mdadm --create /dev/md0 --auto=yes --level=5 --raid-devices=3 --chunk=64 /dev/sdb{2,3,4}
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[root@client ~]# mkfs.xfs /dev/md0
meta-data=/dev/md0               isize=512    agcount=8, agsize=65456 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=0, sparse=0
data     =                       bsize=4096   blocks=523648, imaxpct=25
         =                       sunit=16     swidth=32 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=16 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
[root@client mnt]# mkdir raid_md0
[root@client mnt]# mount /dev/md0 /mnt/raid_md0/
[root@client mnt]# df -h /dev/md0
Filesystem      Size  Used Avail Use% Mounted on
/dev/md0        2.0G   33M  2.0G   2% /mnt/raid_md0

check the status

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[root@client ~]# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 sdb4[3] sdb3[1] sdb2[0]
      2095104 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
      
unused devices: <none>
[root@client ~]# mdadm --detail /dev/md0
/dev/md0:
        Version : 1.2
     Raid Level : raid5
     Array Size : 2095104 (2046.00 MiB 2145.39 MB)
  Used Dev Size : 1047552 (1023.00 MiB 1072.69 MB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

          State : clean 
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           Name : client.centos7.study:0  (local to host client.centos7.study)
           UUID : add4f325:d589a29e:1f234b1b:309885ff
         Events : 18

    Number   Major   Minor   RaidDevice State
       0       8       18        0      active sync   /dev/sdb2
       1       8       19        1      active sync   /dev/sdb3
       3       8       20        2      active sync   /dev/sdb4

If mount md0 permanently, need to know md0 UUID

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[root@client mnt]# mdadm  --detail /dev/md0  |grep -i uuid
           UUID : add4f325:d589a29e:1f234b1b:309885ff
[root@client mnt]# vim /etc/mdadm.conf
ARRAY /dev/md0 UUID : add4f325:d589a29e:1f234b1b:309885ff
[root@client mnt]# blkid /dev/md0
/dev/md0: UUID="acf71d8f-8027-49fc-b3dd-fe6d89f3491f" TYPE="xfs"
[root@client mnt]# vim /etc/fstab
UUID="acf71d8f-8027-49fc-b3dd-fe6d89f3491f"    /mnt/raid_md0  xfs defaults  0 0
[root@client mnt]# mount -a

Close raid

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[root@client mnt]# umount /dev/md0
[root@client ~]# mdadm --manage /dev/md0 --fail /dev/sdb2
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: set /dev/sdb2 faulty in /dev/md0
[root@client ~]# mdadm --manage /dev/md0 --fail /dev/sdb3
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: set /dev/sdb3 faulty in /dev/md0
[root@client ~]# mdadm --manage /dev/md0 --fail /dev/sdb4
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: set /dev/sdb4 faulty in /dev/md0
[root@client ~]# mdadm --detail /dev/md0
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
/dev/md0:
        Version : 1.2
  Creation Time : Wed Jul 17 11:37:14 2019
     Raid Level : raid5
     Array Size : 2095104 (2046.00 MiB 2145.39 MB)
  Used Dev Size : 1047552 (1023.00 MiB 1072.69 MB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Wed Jul 17 12:53:53 2019
          State : clean, FAILED 
 Active Devices : 0
 Failed Devices : 3
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       -       0        0        1      removed
       -       0        0        2      removed

       0       8       18        -      faulty   /dev/sdb2
       1       8       19        -      faulty   /dev/sdb3
       3       8       20        -      faulty   /dev/sdb4
[root@client ~]# mdadm --manage /dev/md0 --remove /dev/sdb2
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: hot removed /dev/sdb2 from /dev/md0
[root@client ~]# mdadm --manage /dev/md0 --remove /dev/sdb3
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: hot removed /dev/sdb3 from /dev/md0
[root@client ~]# mdadm --manage /dev/md0 --remove /dev/sdb4
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: hot removed /dev/sdb4 from /dev/md0
[root@client ~]# mdadm --detail /dev/md0
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
/dev/md0:
        Version : 1.2
  Creation Time : Wed Jul 17 11:37:14 2019
     Raid Level : raid5
     Array Size : 2095104 (2046.00 MiB 2145.39 MB)
  Used Dev Size : 1047552 (1023.00 MiB 1072.69 MB)
   Raid Devices : 3
  Total Devices : 0
    Persistence : Superblock is persistent

    Update Time : Wed Jul 17 12:55:39 2019
          State : clean, FAILED 
 Active Devices : 0
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       -       0        0        1      removed
       -       0        0        2      removed
[root@client ~]# mdadm --stop /dev/md0
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: stopped /dev/md0
[root@client ~]# mdadm --detail /dev/md0
mdadm: only give one device per ARRAY line: /dev/md0 and UUID
mdadm: only give one device per ARRAY line: /dev/md0 and :
mdadm: only give one device per ARRAY line: /dev/md0 and add4f325:d589a29e:1f234b1b:309885ff
mdadm: ARRAY line /dev/md0 has no identity information.
mdadm: cannot open /dev/md0: No such file or directory
[root@client ~]# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
unused devices: <none>
[root@client ~]# lsblk
NAME                         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdb                            8:16   0 29.9G  0 disk 
├─sdb1                         8:17   0    2G  0 part 
│ ├─thin_vg-thinpool_tmeta   253:4    0    4M  0 lvm  
│ │ └─thin_vg-thinpool-tpool 253:6    0    1G  0 lvm  
│ │   ├─thin_vg-thinpool     253:7    0    1G  0 lvm  
│ │   └─thin_vg-annathin1    253:8    0   10G  0 lvm  
│ └─thin_vg-thinpool_tdata   253:5    0    1G  0 lvm  
│   └─thin_vg-thinpool-tpool 253:6    0    1G  0 lvm  
│     ├─thin_vg-thinpool     253:7    0    1G  0 lvm  
│     └─thin_vg-annathin1    253:8    0   10G  0 lvm  
├─sdb2                         8:18   0    1G  0 part 
├─sdb3                         8:19   0    1G  0 part 
└─sdb4                         8:20   0    1G  0 part 
sdc                            8:32   0   40G  0 disk 
sr0                           11:0    1 1024M  0 rom  
[root@client ~]# vim /etc/mdadm.conf
#ARRAY /dev/md0 UUID : add4f325:d589a29e:1f234b1b:309885ff
[root@client ~]# vim /etc/fstab
#UUID="acf71d8f-8027-49fc-b3dd-fe6d89f3491f"    /mnt/raid_md0  xfs defaults  0 0