fstrim has no effect on a just-mounted filesystem

["Richard W.M. Jones" <rjones@redhat.com>]

Here's a problem I can't work out:

I have a filesystem (in a VM) that I know has at least 100MB of
deleted files on it.  Doing this in a script:

  mount -o discard /dev/sda1 /mnt
  fstrim /mnt

... does nothing.  Also the fstrim is almost instantaneous -- there's
no way it could be scanning the disk.

However, if I start with the same filesystem, mounted with -o discard,
and create and rm large files, while observing the size of the
underlying virtual disk, then discard is obviously working fine.  'rm'
of large files makes the underlying disk shrink.

Any ideas here?

Rich.

kernel: 3.12.5-302.fc20.x86_64
qemu: 1.7.0
virtio-scsi with discard=unmap

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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