Explain role of logical volume manager(LVM) in Linux Kernel

Mumbai University > Information Technology > Sem 5 > Open Source Technology

Marks: 10M

Year: Dec 2015

1 Answer

Logical volumes are a flexible method for organizing disk storage. They provide benefits over the use of partitions, for example, in that it is much easier to increase or decrease a logical volume in size than a partition. In Linux, Logical Volume Manager (LVM) is a device mapper target that provides logical volume management for the Linux kernel. Most modern Linux distributions are LVM-aware to the point of being able to have their root file systems on a logical volume.

LVM is used for the following purposes:

  • Creating single logical volumes of multiple physical volumes or entire hard disks (somewhat similar to RAID 0, but more similar to JBOD), allowing for dynamic volume resizing.
  • Managing large hard disk farms by allowing disks to be added and replaced without downtime or service disruption, in combination with hot swapping.
  • On small systems (like a desktop), instead of having to estimate at installation time how big a partition might need to be, LVM allows filesystems to be easily resized as needed.
  • Performing consistent backups by taking snapshots of the logical volumes.

LVM can be considered as a thin software layer on top of the hard disks and partitions, which creates an abstraction of continuity and ease-of-use for managing hard drive replacement, repartitioning and backup.

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Basic functionality

  • Volume groups (VGs) can be resized online by absorbing new physical volumes (PVs) or ejecting existing ones.
  • Logical volumes (LVs) can be resized online by concatenating extents onto them or truncating extents from them.
  • LVs can be moved between PVs.
  • Creation of read-only snapshots of logical volumes (LVM1), or read-write snapshots (LVM2).
  • VGs can be split or merged in situ as long as no LVs span the split. This can be useful when migrating whole LVs to or from offline storage.
  • LVM objects can be tagged for administrative convenience.[6]
  • VGs and LVs can be made active as the underlying devices become available through use of the lvmetad daemon

Advanced functionality

  • Hybrid volumes can be created using the dm-cache target, which allows one or more fast storage devices, such as flash-based SSDs, to act as a cache for one or more slower hard disk drives.
  • Thinly provisioned LVs can be allocated from a pool.
  • On newer versions of device mapper, LVM is integrated with the rest of device mapper enough to ignore the individual paths that back a dm-multipath device if devices/multipath_component_detection=1 is set in lvm.conf. This prevents LVM from activating volumes on an individual path instead of the multipath device.
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