Univers Libre

How to enlarge your raw disk image

Written on 10 September 2015, 17:04 CEST
Tags: sysadmin.

Extending a disk image file (of a KVM guest for example) is a tricky operation, especially if we haven't enough disk space to keep a backup of the image we will working on. So I write a little howto for the next time we have to do this operation.


First, you need to have the following packages installed: qemu-utils kpartx parted.

Make sure your VM is halted and no processes have opened the file:

# lsof $img

Note: for following commands, I assume $img contains the path to your image file.

Extend the image file

Pretty easy step, you can simply use qemu-img tool:

# qemu-img resize $img +50G
Image resized.

You can mount the image and check that it has the correct size:

# kpartx -v -a $img
add map loop0p1 (254:13): 0 195318207 linear /dev/loop0 63
# fdisk -l /dev/loop0 

Disk /dev/loop0: 214.7 GB, 214748364800 bytes

Extend the partition

Then, you have to extend the partition. That supposes of course either the disk image contains only one partition (which could be a LVM PV) or the partition you want to extend is at the end of the image.

Before all, backup your partition table:

# sfdisk -d /dev/loop0 >~/loop0.parts

(It could be restored with sfdisk /dev/loop0 <~/loop0.parts).

And remove and recreate the partition using all free space with parted:

# parted /dev/loop0 print
Model:  (file)
Disk /dev/loop0: 215GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End    Size   Type     File system  Flags
 1      32.3kB  100GB  100GB  primary  ext3
# parted /dev/loop0 rm 1
# parted /dev/loop0 mkpart primary ext3 32.3kB 215GB
# parted /dev/loop0 print

Remount the image to see her new size:

# kpartx -d $img
loop deleted : /dev/loop0
# kpartx -v -a $img
add map loop0p1 (254:13): 0 419430337 linear /dev/loop0 63

Extend the filesystem

We are almost there, remaining the filesystem. First, this is a good thing to run a fsck before all:

# e2fsck -f /dev/mapper/loop0p1 
e2fsck 1.42.5 (29-Jul-2012)
/dev/mapper/loop0p1: recovering journal
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/mapper/loop0p1: 41753/6111232 files (9.1% non-contiguous), 23479003/24414775 blocks

Then, resize the filesystem:

# resize2fs /dev/mapper/loop0p1 
resize2fs 1.42.5 (29-Jul-2012)
Resizing the filesystem on /dev/mapper/loop0p1 to 52428792 (4k) blocks.
The filesystem on /dev/mapper/loop0p1 is now 52428792 blocks long.

It's done! We can check if we see the correct size after mounting the partition:

# mount /dev/mapper/loop0p1 /mnt
# df -h /mnt
Filesystem           Size  Used Avail Use% Mounted on
/dev/mapper/loop0p1  197G   89G  109G  45% /mnt
# umount /mnt

Finally, unmount the image:

# kpartx -d $img
loop deleted : /dev/loop0