fstrim on 30 GB ntfs partition says 16 EiB (18446744073709551615 bytes) trimmed

You will have to provide more detailed information on what command you ran and how you mounted the filesystem on which you ran it. When I try to run fstrim an an ntfs filesystem, I just get "fstrim: /mnt: the discard operation is not supported".

I use :
sudo fstrim -va

drive is mounted automatically on startup with this fstab standard parameters :
/mnt/PLX2 auto nosuid,nodev,nofail,x-gvfs-show 0 0

The -a means to run on all mounted filesystems, so the error could be from any of them. Try running it on each filesystem and see which one is causing the message ( which likely isn't your ntfs partition ).

No it couldn't because -v (verbose) option is telling exactly what fstrim has done on what partition.

Did you not show all of the output? That is, does it specifically say that error happened while trimming the ntfs partition?

Ahh, I was able to reproduce this with a loop mounted ntfs image. Looks like a bug in the ntfs-3g driver.

Proposed patch for reporting the real trimmed size attached

The attachment "Proposed patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

How to apply this ?

Maybe this is not ntfs-3g driver bug. I have just compiled latest stable version of ntfs-3g 2015.3.14 (March 31, 2015) and the issue is still present
In Ubuntu 15.10 default ntfs-3g driver version is: 2014.2.15AR.3-3

> Maybe this is not ntfs-3g driver bug.
The bug is present in all ntfs-3g versions earlier than Nov 23, 2015.
The bug is only about the reported trimming count when using the option -v. The trimming operation is not affected by the bug, so you probably need not apply the patch if you do not know how to.

Status changed to 'Confirmed' because the bug affects multiple users.