The green led (ACT) on my Raspberry Pi 3 is set to mmc0 to flash on SD card activity via

echo mmc0 |sudo tee /sys/class/leds/led0/trigger

However, it flashes much more often (randomly every few seconds) than it should according to

iotop -bktoqqq -d .5


iostat -dzp 5

So I wonder what is the light indicating if nothing is read or written to the SD card for many seconds or minutes.

Has anybody seen this LED off for longer periods in the mmc0 mode or does it always flash unrelated to SD card I/O on a Raspberry Pi 3?

It also has an mmc1 mode, in which it flashes even more frequently. I wonder what that indicates.

Why does it matter? After a colleague pointed out that Raspberries kill SD cards, I wanted to reduce my writes to my SD card in order to increase its lifetime and was expecting a less flashing LED indicating some sort of success. Now I am worried that the controller is still messing with it.

  • 1
    The SD card is read twice a second to check it hasn't been removed (it will be a very brief flash, almost unnoticeable to most people). Is that what you are seeing?
    – joan
    Commented Feb 12, 2017 at 23:48
  • The normal way of setting trigger is a config.txt like dtparam=act_led_trigger=heartbeat, although mmc0 should be the default. Modifying the file the way you did leaves a non-standard file which would normally indicate trigger like [mmc0] (among other entries). The contents vary between Pi models.
    – Milliways
    Commented Feb 13, 2017 at 1:21
  • @joan No, that is not the flash I am seeing. It is a random flashing of some flashes per second. Commented Feb 13, 2017 at 17:54
  • @Milliways I did not modify any file. [mmc0] is also the default on my system. The first command is just there to make it clear which mode I am talking about. Commented Feb 13, 2017 at 17:57
  • I doubt it's an error so you may need to dig deeper to find out what is accessing the SD card.
    – joan
    Commented Feb 13, 2017 at 18:00

1 Answer 1


I have also asked this question Why is the green LED (ACT) blinking without SD card activity also at the Raspberry Pi Forum and received this answer:

by rpdom » Wed Feb 22, 2017 8:02 pm

From what I understand, the Pi sends a "status" request to the card every few seconds to check that it is still there and working. That will cause the LED to flash, but won't cause any data to be read or written to the card itself, just the controller on the card. It won't cause wear.

I have not had a Raspberry kill an SD card yet, and I have several of them, some of which have been running for a few years.

But also questions about the mmc1 mode and documentation remain.

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