mdadm

mdadm is a Linux utility used to manage software RAID arrays implemented by the kernel's md (multiple device) driver. It provides commands to create, assemble, monitor, and maintain arrays, enabling redundancy and performance without a dedicated hardware RAID controller.

Common operations include creating new arrays, assembling existing ones found on disk, adding or removing member devices, replacing failed disks, and reshaping or resynchronizing arrays while they are online. mdadm also supports hot spares and automatic assembly at boot by reading its configuration.

mdadm supports several RAID layouts, including RAID 0, 1, 4, 5, 6 and 10, as well as linear configurations. Arrays carry metadata describing their layout; mdadm supports multiple metadata versions, such as 0.90, 1.0, 1.1, 1.2, and 1.3, which may be stored on different offsets on the member disks.

Status and configuration are exposed through the device files /dev/mdX and the proc file /proc/mdstat. The configuration can be written to /etc/mdadm/mdadm.conf (or /etc/mdadm.conf on some systems) and used by boot-time tooling to assemble arrays automatically.

Administration and monitoring can be performed with commands such as mdadm --detail, mdadm --examine, and mdadm --monitor. The --monitor mode can alert administrators to failures or resync progress via syslog or email; log and configuration handling varies by distribution.

mdadm is a core part of Linux software RAID and is widely used in servers and desktops. It is kernel-based and independent of a particular hardware RAID controller, but it requires initramfs support to assemble arrays at boot on some systems. It is compatible with most common filesystems and can be used beneath modern filesystems as well as with virtualization environments.