Android - When should fstrim run?

The article referenced in @mattm's answer is a bit outdated. IdleMaintenanceService was removed in Lollipop, things have changed. On Pie, scheduled fstrim is triggered from StorageManagerService ⁽¹⁾ to MountService ⁽²⁾:

if the device is alive at midnight + idle, fstrim gets run at the next available charging + idle time

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Mount service calls StorageManager which in turn calls Vold to do fstrim.

If the schedule is being missed because the conditions don't meet and fstrim has not run for last 3 days ⁽⁴⁾, it will be forced during device startup if the device is restarted. SystemServer ⁽⁵⁾, PackageManager ⁽⁶⁾, StorageManager ⁽⁷⁾ and Vold ⁽⁸⁾ are involved.

In each case, the file /data/system/last-fstrim ⁽⁹⁾ (since Lollipop) gets updated ⁽¹⁰⁾, so you can check its timestamp (requires root) to find out when fstrim was last run.

If you want to do TRIM manually, use Android's commandline tool sm fstrim (requires root or adb shell). Previously this was vdc fstrim dotrim but added to Storage Manager in Oreo ⁽¹¹⁾ and removed from vold in Pie ⁽¹²⁾.

In order to send FITRIM ioctl to selected filesystem(s) only, use fstrim tool (requires root); a busybox applet.

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What if you don't have root?
And you suspect that the scheduled fstrim is being missed for 3+ days, restart device, fstrim will be forced (since Lollipop 5.1 ⁽¹³⁾). Simple.

Apps can't TRIM without root; see this answer.

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HOW OFTEN TRIM SHOULD BE RUN?

Flash memory (including SSDs, eMMCs, SD Cards, USB sticks etc.) has limited number of Program/Erase cycles before it dies. But unlike HDDs, data can't be just overwritten on flash memory, it has to be Erased first, which is achieved through Garbage Collection.

Now if we do too much fstrim, this will cause frequent GC in background consuming precious P/E cycles and hence reducing life. If we don't do fstrim at all, there could be unnecessary GC (of already deleted data), plus the write operations will get too slow with time because Erase will have to be performed before overwriting data, which has very higher latency as compared to Program operation.

So we need a balance between both.

Also there are other factors e.g. whether eMMC controller supports over provisioning, how effective it is in background GC scheduling, wear-leveling and managing write amplification etc.

So unless you have the data sheet of your phone's eMMC and you are sure what you are doing, better is to leave the fstrim to OS. On PCs' ⁽¹⁴⁾:

Running fstrim frequently, or even using mount -o discard, might negatively affect the lifetime of poor-quality SSD devices. For most desktop and server systems a sufficient trimming frequency is once a week.

But this may vary with situations.

RELATED:

• Coding for SSDs
• Why is TRIM not supported on encrypted “/data” partition?

Unforgettableid's comment included an AnandTech article that describes the conditions under which fstrim is supposed to run:

I’ve learned a bit more on the conditions underlying when Android 4.3 will TRIM filesystems, as it wasn’t completely clear before. The Android framework will send out a “start idle maintenance window” event that the MountService listens for, and then invokes vold to fstrim filesystems when a few conditions have been met – the device hasn’t been touched for over an hour, no idle maintenance window event has been sent in 24 hours, and the device is either off-charger with 80% battery or on-charger with 30% battery. The goal is to have fstrim run roughly once every 24 hours if you’re in the habit of plugging the device in to charge every night.