Understanding and Managing LVM Components

LVM (Logical Volume Manager) introduces an abstraction layer over physical storage, enabling flexible volume management.

• Physical Volume (PV): A block device—such as a disk partition or RAID array—formatted for use by LVM.
• Volume Group (VG): A pool of storage created by combining one or more PVs. The VG is subdivided into fixed-size units called Physical Extents (PEs).
• Logical Volume (LV): A virtual partition carved from the VG’s PEs. LVs can be formattted with a filesystem and mounted like regular partitions. Their size can be resized dynamically without data loss, provided sufficient space exists in the VG.
• Physical Extent (PE): The smallest allocatable unit within a VG. All LVs are composed of PEs.

Key Management Commands

Physical Volume Tools

• pvs: Brief PV summary
• pvdisplay: Detailed PV info
• pvcreate /dev/device: Initialize a device as a PV

Volume Group Tools

• vgs: Brief VG summary
• vgdisplay: Detailed VG info
• vgcreate [-s PE_size] vg_name /dev/device...: Create a VG (default PE = 4 MiB)
• vgextend vg_name /dev/device: Add a PV to a VG
• pvmove /dev/device_to_remove: Migrate data off a PV before removal
• vgreduce vg_name /dev/device: Remove a PV from a VG
• vgremove vg_name: Delete a VG

Logical Volume Tools

• lvs: Brief LV summary
• lvdisplay: Detailed LV info (path: /dev/vg_name/lv_name)
• lvcreate -L size -n lv_name vg_name: Create an LV
• lvremove /dev/vg_name/lv_name: Delete an LV

Resizing Logical Volumes

Extending an LV (ext4 example):

umount /dev/vg_name/lv_name
lvextend -L +1G /dev/vg_name/lv_name
resize2fs /dev/vg_name/lv_name
mount /dev/vg_name/lv_name /mount/point

Shrinking an LV (ext4 only; insure data fits in new size):

umount /dev/vg_name/lv_name
e2fsck -f /dev/vg_name/lv_name
resize2fs /dev/vg_name/lv_name 2G
lvreduce -L 2G /dev/vg_name/lv_name
mount /dev/vg_name/lv_name /mount/point

Note: resize2fs works only with ext2/3/4. For XFS, use xfs_growfs; XFS cannot be shrunk.

Practical Example

Assume three 5 GiB partitions (/dev/sda5, /dev/sda6, /dev/sda7) with type 8e (Linux LVM):

1. Initialize PVs:

   pvcreate /dev/sda5 /dev/sda6 /dev/sda7
   

2. Create a VG named storage_vg with 8 MiB PEs:

   vgcreate -s 8M storage_vg /dev/sda5 /dev/sda6
   

3. Add another PV later:

   vgextend storage_vg /dev/sda7
   

4. Create a 2 GiB LV named data_lv:

   lvcreate -L 2G -n data_lv storage_vg
   

5. Format and mount:

   mkfs.ext4 /dev/storage_vg/data_lv
   mkdir -p /mnt/data
   mount /dev/storage_vg/data_lv /mnt/data
   

6. Enable automatic mounting at boot: Add to /etc/fstab:

   /dev/storage_vg/data_lv  /mnt/data  ext4  defaults  0 0
   

Using dd for Low-Level Operations

The dd command copies and converts files at the byte level.

Common uses:

• Disk cloning: dd if=/dev/sda of=/dev/sdb
• Backup MBR: dd if=/dev/sda of=mbr.img bs=512 count=1
• Wipe bootloader: dd if=/dev/zero of=/dev/sda bs=446 count=1

Special devices:

• /dev/zero: Produces a continuous stream of null bytes.
• /dev/null: Discards all data written to it.