lv extension | lvm extend pv

Logical Volume Manager (LVM) is a powerful and flexible storage management system that provides a layer of abstraction between physical storage devices and the filesystems that applications use. This abstraction allows for dynamic resizing of storage, easy snapshots, and other advanced features. A crucial component of LVM is the ability to extend logical volumes (LVs), allowing you to increase their size as your data storage needs grow. This article provides a comprehensive guide to using `lvextend`, the command-line tool used to extend LVs, covering various scenarios and best practices. We'll also delve into related topics such as extending filesystems after LV extension, how to extend LVM to fill a disk, considerations for snapshot LVs, and specific instructions for CentOS 7.

Understanding `lvextend`

The `lvextend` command is the primary tool for increasing the size of an existing logical volume. It adds unallocated space from the volume group (VG) to the LV, thereby increasing its capacity. This space typically comes from unallocated physical extents (PEs) within physical volumes (PVs) that belong to the VG.

The basic syntax of `lvextend` is:

```bashlv extension

lvextend [options] LogicalVolumePath

Where `LogicalVolumePath` is the path to the logical volume you wish to extend, typically in the form `/dev/mapper/VolumeGroupName-LogicalVolumeName` or `/dev/VolumeGroupName/LogicalVolumeName`.

Common `lvextend` Options:

* `-l, --extents +[Number]PEs[%VG|%FREE]`: This option specifies the number of physical extents to add to the LV. You can specify an absolute number of PEs (e.g., `+100`), a percentage of the volume group's total size (`+%VG`), or a percentage of the volume group's free space (`+%FREE`). This is often the most convenient and flexible way to extend LVs. For example, `lvextend -l +100%FREE /dev/vg01/lv01` will extend lv01 to consume all available space in vg01.

* `-L, --size +[Size][Units]`: This option specifies the amount of space to add to the LV in specific units like kilobytes (K), megabytes (M), gigabytes (G), terabytes (T), or petabytes (P). For example, `lvextend -L +10G /dev/vg01/lv01` will add 10 gigabytes to lv01. Using the `+` sign is crucial; omitting it will set the LV to the specified size, potentially truncating data.

* `-r, --resizefs`: This crucial option automatically resizes the filesystem after the LV has been extended. This is often the desired behavior as it ensures the filesystem can utilize the newly added space. Using this option simplifies the process and reduces the risk of errors. This option only works if the underlying filesystem supports online resizing. For most common filesystems like ext4 and XFS, this works seamlessly.

* `-n, --nofsresize`: Prevents the filesystem from being automatically resized. You would use this option if you intend to resize the filesystem manually later or if the filesystem does not support online resizing.

* `-v, --verbose`: Provides detailed output, which can be helpful for troubleshooting.

* `-f, --force`: Forces the extension even if there are potential issues. Use with caution.

* `-t, --test`: Performs a dry run, showing what would happen without actually making any changes.

* `--alloc AllocationPolicy`: Specifies the allocation policy to use when selecting physical extents. Common policies include `normal`, `contiguous`, and `anywhere`. The `normal` policy is usually sufficient.

Extending LVM to Fill Disk: `lvcreate 100%FREE` and `lvextend -l +100%FREE`

A common requirement is to extend an LVM setup to utilize the entire available space on a disk. This involves two primary scenarios:

1. Creating a new LV that utilizes all free space during creation: This is achieved using the `lvcreate` command with the `-l 100%FREE` option. For example:

```bash

lvcreate -l 100%FREE -n mylv myvg

```

This command creates a new LV named `mylv` within the volume group `myvg` and allocates all available free space in the VG to it. This is typically done during the initial LVM setup or when adding new physical volumes to an existing VG.

2. Extending an existing LV to consume all remaining free space: This is accomplished using `lvextend` with the `-l +100%FREE` option. For example:

```bash

lvextend -l +100%FREE /dev/myvg/mylv

```

This command extends the LV `mylv` in the volume group `myvg` to utilize all currently unallocated physical extents within the VG. It's important to use the `+` sign to *add* to the existing size rather than *set* the size.