mdadm

Mdadm ( for multiple disk administration, formerly mdctl ) is a utility for Linux to manage a software RAID. The program RAID sets can be created, configured, monitored and will be deleted. mdadm is released as free software under the GNU General Public License ( GPL).

  • 2.1 Functionality

Features

Mdadm created so-called multiple devices (short MD) from different block devices (such as an entire disk, a single partition or a USB stick ). The MD can then in turn be used by the MD device driver as a block device. mdadm supports the following types of discs:

  • Linear: concatenating multiple partitions
  • Multipath: No RAID, but a mapping of a file on two different paths on the same partition (mirroring)
  • Faulty: Emulates a faulty RAID system for test cases
  • Level 0 ( block-level striping): concatenation of several small block devices to a large.
  • Level 1 (Mirror ): Mirroring a disk
  • Level 4: Same as Level 0, but with an additional device for parity bits (increased reliability ).
  • Level 5: Level 4, the parity bits are but spread over all devices.
  • Level 6: How to Level 5 but with two independent parity bits per segment (increased reliability ).
  • Level 10: Combination of Level 0 and Level 1

Learn more about the different RAID levels can also be found in the article RAID # The common RAID levels in detail.

Could Before the Linux 2.6 kernel mdadm MDs as / dev / mdn ( where n is a number between 0 and 99 ) can be included, the 2.4 kernel as / dev / md / name, where name is a name of your choice. These devices were not partitioned. The kernel 2.6. introduced new partitionable arrays. The devices are now called / dev/md_d2p3 ( the third partition of the second MDs ). Since kernel 2.6.28, the old discs can be partitioned. They are then called / dev/md2p3 ( the third partition of the second MDs ).

Basically, the problem is when booting a kernel, which is on a MD is the fact that the BIOS or the boot loader there is no support for the use RAID algorithms. Meanwhile, with GRUB2 and its modules, the opportunity has been created, a simple software RAID to provide support in the bootloader. So it is now also possible to boot a kernel from an LVM volume from a soft raid. The boot loader and its modules are doing for MBR partitioned disks in the first sectors and partitioned with GPT in a separate partition.

Mdmpd

Mdmpd is a computer program for GNU / Linux operating systems. It is part of the mdadm package and was originally introduced by the company Red developed.

The program is loaded at boot time as a daemon and then used to monitor RAID arrays. The name of the program mdmpd is synonymous with engl. daemon to monitor MD multipath devices (German background service to monitor multiple transmissions on hard disks).

Storage solutions in organizations often need multiple ways to communicate with a single drive within a RAID system. This enables the system in case of failure of the controller to which a drive is attached to easily communicate with the drive on a different controller do and thus remains fully functional. This solution is also called multipath disk access ( disk access through multiple channels ) and used for example in SAS hard drives. The implementation of this access via the software RAID functionality of the Linux kernel, as md driver (multiple disk) known.

Operation

The part of the md driver in the kernel does only I / O requests to the appropriate drive and the error handling of the active channel. A verification that a formerly faulty channel is functional again, does not take place.

At system startup, the daemon performs an initial review of the RAID volumes and storage of this state, then the daemon runs in the background and waits for further notifications from the kernel.

After receiving such a message, the daemon checks whether the active channel is down to a drive. After that every 15 seconds a request sent through this channel until the failure has been corrected by changing the channel. Once the fault is removed, the daemon adds the ( no longer active ) channel the original RAID as a backup channel added.

When using procfs, a dynamic file system for storing system-relevant data, in the virtual file / proc / mdstat all configured RAID sets and other information concerning the condition stored. mdmpd needs this information to locate the RAID system, to identify the components of the RAID volumes as well as for notification of relevant events.

560723
de