Bsrgvty

I know that weekly TRIM is enabled by default from 14.10 onwards. Source: How to enable TRIM?
But running sudo nano /etc/cron.weekly/fstrim returns an empty file. Also tail -n1 /etc/cron.weekly/fstrim says that this file does not exist.

Running lsblk -D returns non zero values for DISC-GRAN and DISC-MAX
so TRIM is supported on my SSD. Is weekly TRIM actually enabled for my SSD or not?

I am using a Kingston SSD

Trim (fstrim/discard) is enabled to run weekly by default in 18.04.

It is set to run for all mounted filesystems on devices that support the discard operation.

It is a systemd service managed via systemctl, not CRON.

View fstrim.timer status:

$ systemctl status fstrim.timer
● fstrim.timer - Discard unused blocks once a week
 Loaded: loaded (/lib/systemd/system/fstrim.timer; enabled; vendor preset: enabled)
 Active: active (waiting) since Thu 2018-07-05 05:45:11 BST; 4h 42min ago
 Trigger: Mon 2018-07-09 00:00:00 BST; 3 days left
 Docs: man:fstrim

Start/Stop/Restart fstrim.timer:

(does not change startup status)

$ sudo systemctl [start/stop/restart] fstrim.timer

Enable/Disable fstrim.timer:

(add to/remove from startup, does not change current active status)

$ sudo systemctl [enable/disable] fstrim.timer

View fstrim.timer configuration:

$ systemctl cat fstrim.timer
# /lib/systemd/system/fstrim.timer
[Unit]
Description=Discard unused blocks once a week
Documentation=man:fstrim

[Timer]
OnCalendar=weekly
AccuracySec=1h
Persistent=true

[Install]
WantedBy=timers.target

View fstrim.service configuration:

$ systemctl cat fstrim.service
# /lib/systemd/system/fstrim.service
[Unit]
Description=Discard unused blocks

[Service]
Type=oneshot
ExecStart=/sbin/fstrim -av

Note: ExecStart=/sbin/fstrim -av

Where:
-a, Trim all mounted filesystems on devices that support the discard operation.
-v, Verbose execution. Output the number of bytes passed from the filesystem down the block stack to the device for potential discard.

View related systemd journal entries:

$ journalctl -u fstrim.timer
Jul 04 14:18:41 user-laptop systemd[1]: Started Discard unused blocks once a week.
Jul 04 21:59:26 user-laptop systemd[1]: Stopped Discard unused blocks once a week.
etc...

$ journalctl -u fstrim.service
Jun 25 10:59:44 user-laptop systemd[1]: Starting Discard unused blocks...
Jun 25 10:59:48 user-laptop fstrim[955]: /: 92.5 GiB (99335237632 bytes) trimmed
Jun 25 10:59:48 user-laptop systemd[1]: Started Discard unused blocks.
-- Reboot --
Jul 02 04:27:41 user-laptop systemd[1]: Starting Discard unused blocks...
Jul 02 04:27:46 user-laptop fstrim[1032]: /: 92.3 GiB (99150807040 bytes) trimmed
Jul 02 04:27:46 user-laptop systemd[1]: Started Discard unused blocks.
etc...

Edit: Please read the comments, this answer mixes up two mechanisms!

Old answer

there is an important point which needs to be added to the answer of @Broadsworde to make it complete.

While on my laptop all the timers and services were enabled, the fstrim log entry was missing (only: starting… stopping… reboot… starting… etc.).

Missing step

You might need to mark the file systems as discardable. If a file system is not marked as discardable, the trim will skip it[1].

To mark a filesystem as discardable, you have two options:

1. Option: tune2fs

sudo tune2fs -o discard /dev/mapper/ubuntu--vg-root

This will set the discard option as default for my ext4 device. If you don't use encryption, try /dev/sda instead.

2. Option: /etc/fstab

Make sure to prepend or append the option discard to your existing mount options. For example like this:

/dev/mapper/ubuntu--vg-root / ext4 discard,relatime,errors=remount-ro 0 1

Only after this the timer service will really do something.

Footnotes

• [1] actually, the device needs to support the TRIM operation. But on linux, this is a file system flag. Still, the device the file system is running on needs to support the TRIM operation. To see if your device supports it, use: sudo hdparm -I /dev/sda | grep -i TRIM.