How to Safely Reduce the Size of a Logical Volume

Do not attempt to shrink a volume if the partition is mounted!  Always unmount first! If it is the root volume you need to reduce in size, use a Live DVD/USB instead. You should also take a backup first too 😉

So here we can see I have one Physical Volume, /dev/md0, attached to my one volume group, RAIDVG.

[andy@home-pc ~]$ sudo pvs
  PV         VG     Fmt  Attr PSize PFree
  /dev/md0   RAIDVG lvm2 a--  1.91t    0

We can also see there is zero space left on the volume group.  You can use sudo pvdisplay to see a more detailed output.

As we can see, all this space is being used up by two Logical Volumes.

[andy@home-pc ~]$ sudo lvs
  LV        VG     Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  MediaLV   RAIDVG -wi-ao----   1.32t                                                    
  StorageLV RAIDVG -wi-ao---- 600.00g

I can see the filesystem reports I have 664G available.  I am going to play it nice and safe and only attempt to reduce the size of my LV by 400G.

[andy@home-pc ~]$ sudo df -h /dev/RAIDVG/MediaLV 
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/RAIDVG-MediaLV  1.4T  603G  664G  48% /plex

Don’t forget to unmount!

[andy@home-pc ~]$ sudo umount -v /dev/RAIDVG/MediaLV
umount: /plex (/dev/mapper/RAIDVG-MediaLV) unmounted

Check for potential issues.

[andy@home-pc ~]$ sudo e2fsck -f /dev/RAIDVG/MediaLV
e2fsck 1.43.4 (31-Jan-2017)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/RAIDVG/MediaLV: 35668/88866816 files (11.1% non-contiguous), 163854225/355446784 blocks

It is very important that you run the --reseizefs operand at the same time.

[andy@home-pc ~]$ sudo lvreduce -L -400G /dev/RAIDVG/MediaLV --resizefs
fsck from util-linux 2.29.2
/dev/mapper/RAIDVG-MediaLV: clean, 35668/88866816 files, 163854225/355446784 blocks
resize2fs 1.43.4 (31-Jan-2017)
Resizing the filesystem on /dev/mapper/RAIDVG-MediaLV to 250589184 (4k) blocks.
The filesystem on /dev/mapper/RAIDVG-MediaLV is now 250589184 (4k) blocks long.

  Size of logical volume RAIDVG/MediaLV changed from 1.32 TiB (347116 extents) to 955.92 GiB (244716 extents).
  Logical volume RAIDVG/MediaLV successfully resized.

Note the (minus) -400G. This means reduce by 400G. If I had used 400G instead, LVM would have made the LV 400G, reducing it from 1.3T.  I would have lost over 200G of actual data. Ouch!

And finally I now have 400G available in my VG.

[andy@home-pc ~]$ sudo pvs
  PV         VG     Fmt  Attr PSize PFree  
  /dev/md0   RAIDVG lvm2 a--  1.91t 400.00g

Be careful and remember to take a backup!