How To Configure Periodic TRIM for SSD Storage on Linux Servers

Introduction

The SSD’s internal “garbage collecting” processes are responsible for erasing blocks and managing wear leveling. However, filesystems typically “delete” data by just marking it in their own records as space that is available again. They do not actually erase the data from the underlying storage, but may overwrite the area previously occupied by that data on subsequent writes.

This means that the SSD will typically not know that a page is no longer needed until it receives instructions from the filesystem to write to the same logical location at a later time. It cannot perform its garbage collection routines because it is never informed when data is deleted, just when the space previously reserved for it should now be used for other data.

The TRIM command propagates information about which data is no longer being used from the filesystem down to the SSD. This allows the device to perform its regular garbage collecting duties when idle, in order to ensure that there are zeroed pages ready to handle new writes. The SSD can shuffle data ahead of time, clean up stale pages, and generally keep the device in good working condition.

Performing TRIM on every deletion can be costly, however, and can have a negative impact on the performance of the drive. Configuring periodic TRIM gives the device bulk information about unneeded pages on a regular schedule instead of with each operation.

In this tutorial, you will learn how SSDs and TRIM work and then enable periodic TRIM on a variety of Linux distributions.

Prerequisites

You’ll enable periodic TRIM on three different Linux distributions in this tutorial, Ubuntu 22.04, Debian 11, and CentOS 8. Using the following guides to set up these distributions with a non-root user.

• Initial Server Setup with Ubuntu 22.04
• Initial Server Setup with Debian 11
• Initial Server Setup with CentOS 8

Step 1 - Storing Data on SSDs

To better understand the problems that TRIM solves, it helps to know a few things about how SSDs store and manage their data.

Data Units

Data on SSDs is written and read in units of a fixed size known as pages. Pages, in turn, are grouped together in larger units called blocks.

Read, Write, and Erase Limitations

SSDs can read and write to pages individually. However, they can only erase data at the block level. Another limitation is that writes can only be performed on pages that have been completely zeroed (all bits set to 0). This means that overwriting data directly is impossible.

To modify data, the SSD actually has to read the information from the old location, modify it in memory, and then write the modified data to new, zeroed pages. It then updates an internal table to map the logical location that the operating system is given to the new physical location of the data on the device. The old location is marked in a different internal table as stale: not in use, but not yet zeroed.

Reclaiming Stale Pages

To reclaim the stale pages, the SSD’s internal garbage collecting processes must read all of the valid pages from a block and write them to a new block. Again, the internal table mapping logical and physical locations is updated. The old block, which now contains no unique, still-in-use data can then be zeroed and marked as ready for future writes.

Step 2 - Disabling Continuous TRIM

You may have already enabled continuous TRIM on your devices when they were mounted. Some servers or mounted disks may be automatically configured with continuous TRIM. Before you enable periodic TRIM, it makes sense to take a look at your current mount options.

Continuous TRIM is enabled by mounting a drive or partition with the discard option.

First, find the filesystems that are currently mounted with the discard option:

findmnt -O discard

Output
TARGET     SOURCE    FSTYPE OPTIONS
/mnt/data  /dev/sda1 ext4   rw,relatime,discard,data=ordered
/mnt/data2 /dev/sdb1 ext4   rw,relatime,discard,data=ordered

You can remount these file systems in place, without the discard option, by including -o remount,nodiscard with mount:

sudo mount -o remount,nodiscard /mnt/data
sudo mount -o remount,nodiscard /mnt/data2

If you run the findmnt command again, you should receive no results:

findmnt -O discard

Next, using nano or your favorite text editor, open the /etc/fstab file to see the mount options currently defined for your filesystems. These determine how the filesystems are mounted each boot:

sudo nano /etc/fstab

Look for the discard option and remove it from lines that you find:

. . .
# /dev/sda1 /mnt/data ext4 defaults,nofail,discard 0 0
/dev/sda1 /mnt/data ext4 defaults,nofail 0 0
# /dev/sdb1 /mnt/data2 ext4 defaults,nofail,discard 0 0
/dev/sdb1 /mnt/data2 ext4 defaults,nofail 0 0

Save and close the file when you are finished. If you are using nano, press Ctrl+X, then when prompted, Y and then Enter. The filesystems will now be mounted without the discard option, and will mount in this same way on subsequent boots. We can now set up periodic TRIM for all filesystems that support it.

Step 3 - Setting Up Periodic TRIM for systemd Distributions

Ubuntu 22.04

Ubuntu 22.04 ships with a script that is run weekly by cron. This means that enabling the systemd method described in the following section is unnecessary for Ubuntu 22.04.

To view the script, use cat:

cat /etc/cron.weekly/fstrim

Output
#!/bin/sh
# trim all mounted file systems which support it
/sbin/fstrim --all || true

As you can see, this script needs a version of fstrim with the --all flag. Many versions of 'fstrim` shipped with earlier releases of Ubuntu do not contain this option.

Other systemd Distributions

For other systemd distributions, periodic TRIM can be enabled with the fstrim.timer file, which will run TRIM operations on all capable, mounted drives once a week. This also leverages the fstrim --all option.

At the time of this writing, this is the best method for the following distributions and up:

• Debian 8
• CentOS 7
• Fedora 23
• CoreOS

For CentOS and Fedora, the fstrim.service and fstrim.timer units are available by default. To schedule a weekly TRIM of all attached capable drives, enable the .timer unit:

sudo systemctl enable fstrim.timer

Your server should now TRIM all mounted filesystems that support the operation, once weekly.

Conclusion

Your Linux server should now be configured to periodically TRIM all supported filesystems on a weekly basis. TRIM helps to maximize both the long term performance and lifespan of your SSDs.

Continuous TRIM operations may sound ideal, but they can add significant overhead to regular file system operations. Periodic TRIM offers a good middle ground by relaying key information needed to perform routine maintenance of the drive in a scheduled job instead of as a component of each file operation. Check out our article on Storage Terminology and Concepts in Linux to learn more about this topic.