Qemu images can’t be growed. In this example I will show you a little hack to grow a 6GiB qcow image to a 10GiB image. Beware: these operations can take a lot of time to perform and require lots of free space.

First, convert your qcow image to a plain raw file: 

1$ qemu-img convert system.qcow -O raw system.raw

Then, create a dummy file (filled with zeros) of the size of extra space you want to add to your image. In this case, 4GiB (=10GiB - 6GiB): 

1$ dd if=/dev/zero of=zeros.raw bs=1024k count=4096

Fearlessly, add your extra space to your raw system image: 

1$ cat system.raw zeros.raw > big10G.raw

After that you can boot qemu to verify that added free space is available: 

1$ qemu -hda big10G.raw

Here is an real case example of what you can see in a qemu image on which Windows XP was installed:

Windows disk management inside QEMU showing the grown 10 GB disk, with the 5.99 GB FAT32 system partition and 4.01 GB still unallocated beside it

Now, to grow your primary partition, I suggest you to download a Live CD like gparted Live CD or System Rescue CD , and boot on the .iso file with qemu: 

1$ qemu -hda big10G.raw -cdrom gparted-livecd-0.3.4-5.iso -boot d

This will allow you to grow and manipulate all your partitions safely thanks to parted and other open source system tools.

Finally you can convert back your raw image to a qcow one to not waste space: 

1$ qemu-img convert big10G.raw -O qcow growed-system.qcow

That’s all!

By the way, I think it’s possible to perform the second and third step of this how-to in a single operation using dd only.

Update : I missed it, but this issue is also described in the FAQ from the unofficial #qemu wiki (look at “ How do I resize a disk image? ” question).

18 archived comments

Comments are closed. These were posted on Disqus, and are archived here.

  1. Michael

    Thanks for putting this up. I've tried this a few times but after I cat my raw files together it no longer boots:

    $ qemu winXP_pro-test.img

    - works fine

    $ qemu-img convert winXP_pro-test.img -O raw system.raw
$ qemu system.raw

    - came up to a safe mode prompt the first time, boots normally now.

    $ qemu-img info system.raw
image: system.raw
file format: raw
virtual size: 2.0G (2097152000 bytes)
disk size: 1.0G
$ qemu-img info zeros.raw
image: zeros.raw
file format: raw
virtual size: 6.0G (6442450944 bytes)
disk size: 6.0G
$ cat system.raw zeros.raw > winXP_8G.raw
$ qemu-img info winXP_8G.raw
image: winXP_8G.raw
file format: raw
virtual size: 8.0G (8539602944 bytes)
disk size: 8.0G
$ qemu winXP_8G.raw
Bochs says:  A disk read error has occurred.

    If I run an image image of partedmagic-1.6 it shows the partition correctly and lets me resize it - but resizing a non bootable image doesn't mean a whole lot. My guess is the problem has something to do with the virtual size: 2.0G disk size: 1.0G of my raw file. That's what qemu-img gives me converting from cow format. XP shows the drive as being 2GB when I run the original image.

    1. Kevin Deldycke Author

      I've never encounter your problem during my experiments, and I have no idea how to fix your problem. Sorry...

      I didn't know the qemu-img info command. I don't know if this can help you, but the only raw image file I still have return me this:

      [kevin@localhost qemu]$ qemu-img info qemu-winXP.raw
image: qemu-winXP.raw
file format: raw
virtual size: 10G (10737418240 bytes)
disk size: 9.0G

      As you can see, like you, I have the same 1G offset...

    2. Bruno

      For details on "A disk read error has occurred." please check this post: http://qemu-forum.ipi.fi/viewtopic.php?p=12362

      The complete procedure would now be

      1- Convert to a raw image

      qemu-img convert system.qcow -O raw system.raw

      2- Increase the size of the raw image

      dd if=/dev/zero of=system.raw seek=N obs=1GB count=0

      where N is the number of GigaBytes

      3- Fix the NTFS BPB
      Edit the raw file: hexedit system.raw
      Goto to offset 7E00: 7E00
      Change 80 to FF at offset 7E1A and save: FF --> Ctrl-x --> y

      4- Boot Windows and check that you see a bigger disk in the admin tools

      qemu hda system.raw

      5- Use (fantastic) gparted to resize your NTFS partition

      qemu -hda system.raw -cdrom gparted-livecd-0.3.4-5.iso -boot d

      6- Boot windows to check and perform a chkdsk

      qemu hda system.raw

      7- If you need to save space, convert back to qcow

      qemu-img convert system.raw -O qcow growed-system.qcow
  2. Luca

    If you have the image in a raw file and the host filesystem is supporting files with holes a simpler, quicker and less space-wasting way to increase your virtual disk size is:

    dd if=/dev/zero of=system.raw bs=1 count=1 seek=10G

    In this way dd leaves a hole of 10G from the beginning of the file (leaving everything untouched) and then writes a 0.

    1. Mark

      There's a better way to do the data append than by creating huge files and cat'ing them together...try this to use sparse files and save a bunch of space:

      dd if=/dev/zero of=system.raw bs=1 count=1 seek=$((  - 1 )) conv=notrunc
      1. Brian

        Regarding combining the dd and the cat into one statement, you can change

        dd if=/dev/zero of=zeros.raw bs=1024k count=4096
cat system.raw zeros.raw > big10G.raw

        Into

        dd if=/dev/zero bs=1024k count=4096 >> system.raw

        The absence of an of= parameter makes dd direct its output to STDOUT, which >> then appends to your existing file. I've tested it and it works for me, but as always, make sure you have a backup. In particular, typing > instead of >> would wipe your file.

  3. Anonymous 
    cat system.raw zeros.raw > big10G.raw

    shouldn't that be:

    cat system.raw zeros.raw >> big10G.raw

    Maybe this is why people are having trouble with it?

    1. Stubbs

      The reason that cat system.raw zeros.raw > big10G.raw works is because you're cat'ing the contents of both files before redirecting them to big10G.raw

      In THEORY you should be able to do cat zeros.raw >> system.raw

      but don't quote me on that or blame me if it screws up your image :-)

      1. phil

        I tried

        cat zeros.raw >> system.raw

        and it works!

        At least for me, it's an old linux ext2 image.

        I also tried a gparted twist without the liveCD:

        losetup /dev/loop1 system.raw
gparted /dev/loop1

        it can see it, but cannot resize it :(

        e2label: Bad magic number in superblock ...
    2. cat

      you should really change that cat command, this will overwrite the second file. you need to use >> instead of >

      1. Kevin Deldycke Author

        @cat: no, the cat command is good in my original article. The use of cat there is convoluted, but good. If you're looking for a better cat command, look at those proposed in the comments above.

  4. Porch

    hello!
    Great tutorial, but how i do it in windows?

    1. mjs

      Install a qemu guest with linux and do it :P

      Serious (or not) do not know!

      Great tutorial btw, thanks

    2. tartanandsnarly

      Hi,
      Just to say that I accomplished this on windows by replacing the dd and cat steps - with the windows type command.....

      I created the raw image:

      CMD> qemu-img convert disk.qcow -O raw rawdisk1.raw

      then created a copy of that:

      CMD> copy rawdisk1.raw rawdisk2.raw

      then used that to create a 3rd disk:

      CMD> type rawdisk1.raw rawdisk2.raw > rawdisk3.raw

      rawdisk3 could then boot normally both using qemu (in which case it was in the same condition as rawdisk1) and using the gparted iso - in which case the extra space could be seen.

      I manipulated the partitions using gparted and grew the disk.

      ...as they say in Scotland - nae bother!

  5. Adam

    Probably the easiest way to grow a raw disk image is:

    truncate -size new_image_size disk.raw

    where new_image_size is larger than existing disk.raw.

    1. Capesteve

      @Adam
      -s or --size according to the man page for truncate

      truncate -s +2G BLAHBLAH.img worked for me up to a point.

      I still have to figure out how to grow the root partition of my VM

      1. Capesteve

        I was lucky in that my swap partition was next to my root partition. After increasing raw img size using truncate, I booted into single user mode, used fdisk -cu /dev/vda to edit my partition table. Created new swap partition, deleted old swap partition, deleted root partion, made new partion allowing it to take up all the space. Set boot flag. Wrote table to disk. Ran partx -a /dev/vda to make it active. Then used resize2fs /dev/vda1 to grow the root partition to fill up the space vacated by the old swap partition.

        1. Capesteve

          don't forget to edit the /etc/fstab entry to pint to the new swap file. You can use blkid to get the UUID or just use /dev/vdax where x is the swap partition number.