LVM: Growing a Volume Without Downtime

A full data volume is one of the most common production emergencies. With the Logical Volume Manager (LVM) you can add disk space and grow a mounted filesystem without unmounting it or stopping the application. This post explains the layers, then gives a runbook you can rehearse before you need it.

Versions in use on this date. LVM2 2.02.176 on Ubuntu 18.04.3 LTS. ext4 can be grown online; XFS (the CentOS 7 default) can be grown online but never shrunk. ext4 can be shrunk only while unmounted, so plan sizes with growth in mind.

The three layers

flowchart TB
  D1[Disk /dev/sdb] --> PV1[Physical volume PV]
  D2[New disk /dev/sdc] --> PV2[Physical volume PV]
  PV1 --> VG[Volume group vg_data<br/>pool of extents]
  PV2 --> VG
  VG --> LV1[Logical volume lv_db<br/>mounted at /var/lib/mysql]
  VG --> LV2[Logical volume lv_logs<br/>mounted at /var/log]
  VG --> FREE[Free extents<br/>available to grow any LV]
  LV1 --> FS[Filesystem ext4 or xfs]

A physical volume is a disk or partition given to LVM. A volume group pools one or more PVs. A logical volume is a slice of the pool that holds a filesystem and can be extended later using free space. Growing needs two steps: extend the LV, then grow the filesystem inside it (-r does both).

Read the current state

lsblk -f
sudo pvs; sudo vgs; sudo lvs -a -o +devices
df -hT /var/lib/mysql
sudo vgdisplay vg_data | grep -E 'VG Size|Free'

If VFree in vgs is non-zero, you already have space and can skip straight to extending the LV.

Case 1: free space already in the volume group

sudo lvextend -r -L +20G /dev/vg_data/lv_db          # add 20 GB and grow the filesystem
# or use all free space:
sudo lvextend -r -l +100%FREE /dev/vg_data/lv_db
df -hT /var/lib/mysql

Case 2: add a new disk

lsblk                                              # confirm the new device, for example /dev/sdc
sudo pvcreate /dev/sdc
sudo vgextend vg_data /dev/sdc
sudo lvextend -r -l +100%FREE /dev/vg_data/lv_db

Double-check the device name with lsblk before pvcreate; it destroys any signature on the disk.

Case 3: the cloud disk was enlarged

On a virtual machine you often grow the disk in the provider console instead of adding one. The partition and the PV must be told:

sudo apt install cloud-guest-utils          # provides growpart
sudo growpart /dev/sdb 1                    # grow partition 1 to the new disk size
sudo pvresize /dev/sdb1
sudo lvextend -r -l +100%FREE /dev/vg_data/lv_db

If the filesystem sits directly on the disk without LVM, grow it with sudo resize2fs /dev/sdb (ext4) or sudo xfs_growfs /mountpoint (XFS).

Filesystem commands, when not using -r

sudo resize2fs /dev/vg_data/lv_db            # ext4, online
sudo xfs_growfs /var/lib/mysql               # XFS takes the mount point

Before and after checklist

Take a backup or a provider snapshot first; resizing is safe but a typo is not.
Note current sizes (lvs, df -h) in the ticket.
Run the commands, then confirm the new size with df -h and that the application still writes.
Update monitoring thresholds and capacity notes.

Snapshots and moving data

sudo lvcreate --size 5G --snapshot --name db_snap /dev/vg_data/lv_db
sudo lvremove /dev/vg_data/db_snap          # remove when finished
sudo pvmove /dev/sdb1                       # move extents off a disk before removing it

A snapshot uses copy-on-write space; if it fills up, the snapshot becomes invalid and writes to the origin volume get slow while it exists. Use snapshots for short backup windows, not as long-term history.

Common problems

No free extents: vgs shows 0 free; add a disk (Case 2).
lvextend works but df is unchanged: the filesystem was not grown; run resize2fs or xfs_growfs.
Disk grew but pvs shows the old size: run growpart and pvresize, and rescan with echo 1 | sudo tee /sys/class/block/sdb/device/rescan.
Root volume full: grow it the same way; / on LVM extends online, which is why many installers use LVM by default.