How far is it safe to regularly write to the ESP32 flash, considering the flash MTBF?

Question

What would be the best practice for writing to the flash on a regular basis. Considering the hardware I am working on is supposed to have 10 to 20 years longevity, what would be your recommandation? For example, is it ok I write some state variables every 15 minutes thru Preferences?

Answer 1

That depends on

• number of erase cycles your Flash supports,
• size of the NVS partition where you store data and
• size and structure of the data that you store.

Erase cycles mean how many times a single sector of Flash can be erased before it's no longer guaranteed to work. The number is found in the datasheet of the Flash chip that you use. It's usually 10K or 100K.

Preferences library uses the ESP-IDF NVS library. This requires an NVS partition to store data, the size of which determines how many Flash sectors get reserved for this purpose. Every time you store a value, NVS writes the data together with its own overhead (total of 32 bytes for primitive data types like ints and floats, more for strings and blobs) into the current Flash sector. When the current sector is full, it erases the next sector and proceeds to write there; thereby using up sectors in a round robin fashion as write requests come in.

If we assume that your Flash has 100K erase cycles, size your NVS partition is 128 KiB and you store a set of 8 primitive values (any int or float) every 15 minutes:

1. Each store operation uses 8 * 32 = 256 bytes (32 B per data value).
2. You can repeat that operation 131072 / 256 = 512 times before you've written to every sector of your 128KiB NVS partition (i.e. erased every sector once)
3. You can repeat that cycle 100K times so you can do 512 * 100000 = 51200000 or roughly 5.1M store operations before you've erased every sector its permitted maximum number of times.
4. Considering the interval of 15 minutes creates 365 * 24 * 4 = 35040 operations per year, you'd have 51200000 / 35040 = 1461 years until Flash is dead.

Obviously, if your Flash chip is rated at 10K erase cycles, it drops to only 146 years.

There's probably some NVS overhead in there somewhere that I didn't account for, and the Flash erase cycle ratings are not 100% reliable so I'd cut it in half for good measure - I would expect 700 or 70 years in real life.

If you store non-primitive values (strings, blobs) then the estimate changes based on the length of that data. I don't know to calculate the exact Flash space used by those but I'd guess 32B plus length of your data multiplied by 10% of NVS overhead. Plug in the numbers, see for yourself.