No-copy extracting Xen VM tarballs to LVM

SUSE Studio delivers Xen VM images which is really nice. They contain a sparse image and a (mostly incomplete) VM config file. Since I’m updating them pretty often I needed a hack that saves on any unneeed copies and needs no scratch space, either.

Goal: save copy times and improve life quality instead of copying and waiting…

First, lets have a look at the contents and then let’s check out how to directly extract them…

(Oh. Great. Shitbuntu won’t let me paste here)


Well, great.

I’n my case the disk image is called:


It’s located in a folder named:



So, what we can do is this:

First, set up some variables so we can shrink the command later on…


Then, tie it together to store our VM data.

wget -O- $url | tar -O -xzf - ${folder}/${vmimage} | dd of=$lv bs=1024k

Storing to a file at the same time:

wget -O- $url | tee /dev/shm/myfile.tar.gz | tar -O -xzf - ${folder}/${vmimage} |\
dd of=$lv bs=1024k


Wget will fetch the file, write it to STDOUT, tar will read STDIN, only extract the image file, and write the extracted data to STDOUT, which is then buffered and written by the dd.


If you’ll reuse the image for multiple VMs like me you can also write it to /dev/shm and, if RAM allows, also gunzip it. the gzip extraction is actually limiting the performance, and even tar itself seems to be a little slow. I only get around 150MB/s on this.

I do remember it needs to flatten out the sparse image while storing to LVM, but I’m not sure if / how that influences the performance.


(Of course none of this would be necessary if the OSS community hadn’t tried to ignore / block / destroy standards like OVF as much as they could. Instead OVF is complex, useless and unsupported. Here we are.)


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s