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md assembles multiple devices into a single virtual devivedevice.
The multiple devices are variously called "underlying devices" and "RAID devices" in the documentation. On this WIKI page they are always "underlying devices".
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Create the block devices to be used as underlying devices. Typically these are HDD partitions but any block devices can be used. In the following example, /dev/sdb1 /dev/sdc1 are used.
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mdadm
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--create
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/dev/md0
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--level=1
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--raid-devices=2
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/dev/sdb1
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/dev/sdc1
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The md device may now be used like any other block device except when:
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If the md device provides the file system including /boot or is an LVM PV (physical volume) then /etc/mdadm/mdadm.conf and the initrd must be updated
/usr/share/mdadm/mkconf > /etc/mdadm/mdadm.conf
update-initramfs -u
Replacing an underlying device
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Regardless of how the underlying device was created, it can now be added to the md device. Assuming the underlying device is sdb1 and the md device is md0:
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mdadm
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--manage
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--add
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/dev/md0
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/dev/sdb1
Remove an md device
Removing /dev/md1 as an example ...
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