Centos 7 Zero Free Space

[matthewh]

I have a number of Centos 7 KVM virtual machines using either XFS or EXT4 file system. Is there a safe easy way to ZERO out all unused disk space? I am wanting to make the backups I do of the entire KVM smaller when compressed.

[tunk]

Removed.

[TrevorH]

You can always dd if=/dev/zero of=/tmp/somefile; rm -f /tmp/somefile but there is a window there where something else might request disk space and not be able to obtain it as the filesystem will be full. If you're on a filesystem that has a storage device that supports TRIM then fstrim can discard all unallocated blocks.