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The <filename>/proc/mdstat</filename> file lists existing volumes and their states. When creating a new RAID volume, care should be taken not to name it the same as an existing volume.
We're going to use these physical elements to build two volumes, one RAID-0 and one mirror (RAID-1). Let's start with the RAID-0 volume:
<computeroutput># </computeroutput><userinput>mdadm --create /dev/md0 --level=0 --raid-devices=2 /dev/sdb /dev/sdc
</userinput><computeroutput>mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
# </computeroutput><userinput>mdadm --query /dev/md0
</userinput><computeroutput>/dev/md0: 7.99GiB raid0 2 devices, 0 spares. Use mdadm --detail for more detail.
# </computeroutput><userinput>mdadm --detail /dev/md0
</userinput><computeroutput>/dev/md0:
Version : 1.2
Creation Time : Mon Feb 28 01:54:24 2022
Raid Level : raid0
Array Size : 8378368 (7.99 GiB 8.58 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent

Update Time : Mon Feb 28 01:54:24 2022
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Layout : -unknown-
Chunk Size : 512K

Consistency Policy : none

Name : debian:0 (local to host debian)
UUID : a75ac628:b384c441:157137ac:c04cd98c
Events : 0

Number Major Minor RaidDevice State
0 8 0 0 active sync /dev/sdb
1 8 16 1 active sync /dev/sdc
# </computeroutput><userinput>mkfs.ext4 /dev/md0
</userinput><computeroutput>mke2fs 1.46.2 (28-Feb-2021)
Discarding device blocks: done
Creating filesystem with 2094592 4k blocks and 524288 inodes
Filesystem UUID: ef077204-c477-4430-bf01-52288237bea0
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

# </computeroutput><userinput>mkdir /srv/raid-0
</userinput><computeroutput># </computeroutput><userinput>mount /dev/md0 /srv/raid-0
</userinput><computeroutput># </computeroutput><userinput>df -h /srv/raid-0
</userinput><computeroutput>Filesystem Size Used Avail Use% Mounted on
/dev/md0 7.8G 24K 7.4G 1% /srv/raid-0
</computeroutput>
The <command>mdadm --create</command> command requires several parameters: the name of the volume to create (<filename>/dev/md*</filename>, with MD standing for <foreignphrase>Multiple Device</foreignphrase>), the RAID level, the number of disks (which is compulsory despite being mostly meaningful only with RAID-1 and above), and the physical drives to use. Once the device is created, we can use it like we'd use a normal partition, create a filesystem on it, mount that filesystem, and so on. Note that our creation of a RAID-0 volume on <filename>md0</filename> is nothing but coincidence, and the numbering of the array doesn't need to be correlated to the chosen amount of redundancy. It is also possible to create named RAID arrays, by giving <command>mdadm</command> parameters such as <filename>/dev/md/linear</filename> instead of <filename>/dev/md0</filename>.
Creation of a RAID-1 follows a similar fashion, the differences only being noticeable after the creation:
<computeroutput># </computeroutput><userinput>mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdd2 /dev/sde
</userinput><computeroutput>mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
mdadm: largest drive (/dev/sdc2) exceeds size (4189184K) by more than 1%
Continue creating array? </computeroutput><userinput>y
</userinput><computeroutput>mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.
# </computeroutput><userinput>mdadm --query /dev/md1
</userinput><computeroutput>/dev/md1: 4.00GiB raid1 2 devices, 0 spares. Use mdadm --detail for more detail.
# </computeroutput><userinput>mdadm --detail /dev/md1
</userinput><computeroutput>/dev/md1:
Version : 1.2
Creation Time : Mon Feb 28 02:07:48 2022
Raid Level : raid1
Array Size : 4189184 (4.00 GiB 4.29 GB)
Used Dev Size : 4189184 (4.00 GiB 4.29 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent

Update Time : Mon Feb 28 02:08:09 2022
State : clean, resync
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Consistency Policy : resync

Rebuild Status : 13% complete

Name : debian:1 (local to host debian)
UUID : 2dfb7fd5:e09e0527:0b5a905a:8334adb8
Events : 17

Number Major Minor RaidDevice State
0 8 34 0 active sync /dev/sdd2
1 8 48 1 active sync /dev/sde
# </computeroutput><userinput>mdadm --detail /dev/md1
</userinput><computeroutput>/dev/md1:
[...]
State : clean
[...]
</computeroutput>
<emphasis>TIP</emphasis> RAID, disks and partitions
As illustrated by our example, RAID devices can be constructed out of disk partitions as well, and do not require full disks.
A few remarks are in order. First, <command>mdadm</command> notices that the physical elements have different sizes; since this implies that some space will be lost on the bigger element, a confirmation is required.
More importantly, note the state of the mirror. The normal state of a RAID mirror is that both disks have exactly the same contents. However, nothing guarantees this is the case when the volume is first created. The RAID subsystem will therefore provide that guarantee itself, and there will be a synchronization phase as soon as the RAID device is created. After some time (the exact amount will depend on the actual size of the disks…), the RAID array switches to the “active” or “clean” state. Note that during this reconstruction phase, the mirror is in a degraded mode, and redundancy isn't assured. A disk failing during that risk window could lead to losing all the data. Large amounts of critical data, however, are rarely stored on a freshly created RAID array before its initial synchronization. Note that even in degraded mode, the <filename>/dev/md1</filename> is usable, and a filesystem can be created on it, as well as some data copied on it.
<emphasis>TIP</emphasis> Starting a mirror in degraded mode

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<computeroutput># </computeroutput><userinput>mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdd2 /dev/sde
</userinput><computeroutput>mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
mdadm: largest drive (/dev/sdc2) exceeds size (4189184K) by more than 1%
Continue creating array? </computeroutput><userinput>y
</userinput><computeroutput>mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.
# </computeroutput><userinput>mdadm --query /dev/md1
</userinput><computeroutput>/dev/md1: 4.00GiB raid1 2 devices, 0 spares. Use mdadm --detail for more detail.
# </computeroutput><userinput>mdadm --detail /dev/md1
</userinput><computeroutput>/dev/md1:
Version : 1.2
Creation Time : Mon Feb 28 02:07:48 2022
Raid Level : raid1
Array Size : 4189184 (4.00 GiB 4.29 GB)
Used Dev Size : 4189184 (4.00 GiB 4.29 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent

Update Time : Mon Feb 28 02:08:09 2022
State : clean, resync
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Consistency Policy : resync

Rebuild Status : 13% complete

Name : debian:1 (local to host debian)
UUID : 2dfb7fd5:e09e0527:0b5a905a:8334adb8
Events : 17

Number Major Minor RaidDevice State
0 8 34 0 active sync /dev/sdd2
1 8 48 1 active sync /dev/sde
# </computeroutput><userinput>mdadm --detail /dev/md1
</userinput><computeroutput>/dev/md1:
[...]
State : clean
[...]
</computeroutput>
a year ago
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String age
a year ago
Last updated
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Source string age
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string 105