Finally I tracked down the reason why my early test failed, but then it just started working "by magic".
The reason is this: If you: - create an ext4 filesystem - mount it WITHOUT -o discard - create and remove some big files - unmount - mount -o discard - fstrim then the fstrim has no effect. If you: - create an ext4 filesystem - mount it WITH -o discard - create and remove some big files - unmount - mount -o discard - fstrim then the fstrim works. (Actually it's not necessary, because the previous 'rm' already recovered the space.) I have no idea why this is yet (still investigating). However the fstrim is suspiciously fast, so there's no way it can be scanning the whole disk for deleted files. I don't think fstrim does what it says in the man page. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
