Updating data in flash memory using fstorage.

[soumil]

Hi,

I am using SDK12.3 and stack s132 with NRF52832. We have to store 3 variables in flash memory and keep updating it once BLE connection is formed with the phone. We were using fstorage to read and write the data. It seems like fstorage doesn't update the data but we need to erase the full page first and then write the data again. 

Since these 3 variables are independent of each other and would get updated at different times, should we save them in different pages? If this is correct, then how should we erase the pages individually i.e the second page first and then the first page. 

Looking forward to getting some feedback on this.

[Jørgen Holmefjord]

Hi,

It is not possible to update values in flash without erasing the page first. Bits in flash can only be changed from 1 to 0, not the other way around. Erasing the page will set all bits back to 1.

I recommend that you take a look at FDS (Flash Data Storage). This is a library running on top of fstorage, that implements a minimalistic file system for the flash storage. FDS allows you to update records, by writing a new record before invalidating the old one. When you are running out of space, you can run garbage collection to erase the pages and free more space. This is much more efficient than eraseing the whole page for each flash write, as flash erase operations are quite time consuming. There is an example that show how to use FDS in the latest SDK.

Best regards,
Jørgen

[soumil]

Thanks for the answer. I was just curious to know that if we have three pages and if we want to delete the second page, how can we do that ? Also it is mentioned that fds library is experimental in SDK12.3. Is it still recommended to use it? 

[Jørgen Holmefjord]

When using FDS or fstorage? FDS will handle the pages for you, and will store all records in "virtual pages". When you run garbage collection, valid records will be moved to a swap page before the page is erased. 

You can read more about the experimental tag on this page. Note that fstorage is also marked as experimental in SDK 12.3. Since you are using nRF52832, you can consider migrating to SDK 14.2, where more bugs in fstorage/FDS have been fixed.

[soumil]

Thanks. I actually meant when using fstorage how can we erase individual pages. We will update to SDK 14.2 and use the fds library.

[Jørgen Holmefjord]

The fstorage library documentation provide a code snippet showing how you erase pages. You pass the address of the page you want to erase to the function. I think FDS is a better alternative for your use-case. Note that the number of writes to any location in flash is limited, so it will be beneficial to spread the writes over a larger area than erasing and writing the same address over and over.