Debugging why fstrim doesn't work

[Matthias Schniedermeyer <ms@citd.de>]

Hi


All of my computers have a SSD for the root-filesystem.
But on none fstrim works. I only get this:

$ fstrim -v /
fstrim: /: discard operation not supported.

How can i debug why fstrim/the kernel thinks discard isn't supported?

All my root-filesystems are XFS.
kernel differs by computer, currently between 3.15.2 and 3.16.1
(Same self-compiled kernel on all machines)
util-linux is version 2.25 (Experimental Package from Debian SID)

The mount-output for the root-filesystem of one my compters looks like 
this:
/dev/sda2 on / type xfs (rw,noatime,attr2,inode64,allocsize=64k,noquota)

I DO NOT use an initrd/initramfs, so the root-filesystem is still the 
original mount from the kernel itself (i case that matters)




-- 

Matthias