[linux-lvm] on discards

[Xen]

On discards,

I have a thin pool that filled up when there was enough space, because 
the filesystem hadn't issued discards.

I have ext4 mounted with the discard option, I hope, because it is in 
the list of default mount options of tune2fs:

Default mount options:    user_xattr acl discard

What can I do to ensure that it won't happen again?

Is the filesystem really mounted with discard?

It doesn't say so anywhere.

Regards.