Pb compressing img files


New Member

I've several Xen VMs "img" files, i use to backup then, with a classical "tar gz" and i have no problem, it's very efficient.

For example if I've an 100 Go foo.img file and the system running inside use 10Go space, the result will be a 10Go foo.img.tar.gz file to the max :)

For ALL the img VMs i've, running Debian or CentOS, it works like this.

But I've an odd problem, when i'm compressing ONE specifical img file :

The img file size is 190Go, the system Debian 8.X running Gitab use 15Go, but when i proceed to backup the same way that usually (tar gz etc...) , it create an 140Go tar.gz file, ten times more the real used space.

It's the one and only VMs img who have this behavior, it seems like free space is not compressed.

Is anybody have an idea?



New Member
If the filesystem and device support it then fstrim should be much faster. Also xz -9 usually gives the best compression. It;s also the slowest unless you use -T for multithreading.

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