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Chapter 1. Overview of logical volume management

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Logical volume management (LVM) creates a layer of abstraction over physical storage, which helps you to create logical storage volumes. This provides much greater flexibility in a number of ways than using physical storage directly.

In addition, the hardware storage configuration is hidden from the software so it can be resized and moved without stopping applications or unmounting file systems. This can reduce operational costs.

1.1. LVM architecture

The following are the components of LVM:

Physical volume
A physical volume (PV) is a partition or whole disk designated for LVM use. For more information, see Managing LVM physical volumes.
Volume group
A volume group (VG) is a collection of physical volumes (PVs), which creates a pool of disk space out of which logical volumes can be allocated. For more information, see Managing LVM volume groups.
Logical volume
A logical volume represents a mountable storage device. For more information, see Managing LVM logical volumes.

The following diagram illustrates the components of LVM:

Figure 1.1. LVM logical volume components

LVM Logical Volume Components

1.2. Advantages of LVM

Logical volumes provide the following advantages over using physical storage directly:

Flexible capacity
When using logical volumes, you can aggregate devices and partitions into a single logical volume. With this functionality, file systems can extend across multiple devices as though they were a single, large one.
Convenient device naming
Logical storage volumes can be managed with user-defined and custom names.
Resizeable storage volumes
You can extend logical volumes or reduce logical volumes in size with simple software commands, without reformatting and repartitioning the underlying devices. For more information, see Modifying the size of a logical volume.
Online data relocation

To deploy newer, faster, or more resilient storage subsystems, you can move data while your system is active using the pvmove command. Data can be rearranged on disks while the disks are in use. For example, you can empty a hot-swappable disk before removing it.

For more information on how to migrate the data, see the pvmove man page and Removing physical volumes from a volume group.

Striped Volumes
You can create a logical volume that stripes data across two or more devices. This can dramatically increase throughput. For more information, see Extending a striped logical volume.
RAID volumes
Logical volumes provide a convenient way to configure RAID for your data. This provides protection against device failure and improves performance. For more information, see Configuring RAID logical volumes.
Volume snapshots
You can take snapshots, which is a point-in-time copy of logical volumes for consistent backups or to test the effect of changes without affecting the real data. For more information, see Snapshot of logical volumes.
Thin volumes
Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available physical space. For more information, see Creating and managing thin provisioned volumes (thin volumes).
Cache volumes
A cache logical volume uses a fast block device, such as an SSD drive to improve the performance of a larger and slower block device. For more information, see Enabling caching to improve logical volume performance.

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