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FDS - Max flash storage size, file size and record size

Henrico

Dear Tech Support,

I'm currently working on implementing the Flash Data Storage library and ran in some problems. I'm using SDK 15.3 with softdevice s140 and a custom NRF52840 board.

I got the read, write and delete function working properly and have some problems with understanding the storage size. In the documentation it is stated that the max flash size, that is being used by FDS, is equal to  (FDS_VIRTUAL_PAGES * (FDS_VIRTUAL_PAGE_SIZE * 4)) [bytes]. Default is this set to (3 * (1024 * 4)) = 12.288 bytes. This still makes sense.

In my application I am sending 1 word (4 bytes) every x seconds to the FDS with the same file ID and record key. With record header, this means I'm actually sending 4 words each time, right? Also, before running my application I find and delete all records with the same ID and record key and run garbage collection. So no garbage data is present.

I keep writing 4 words every x seconds until I receive a FDS_ERR_RECORD_TOO_LARGE or FDS_ERR_NO_SPACE_IN_FLASH error. So I expected to receive a FDS_ERR_RECORD_TOO_LARGE error after writing 1024 words (virtual page size). In my case after writing 256 packets of 4 words (3 header + 1 data). However, I never receive a FDS_ERR_RECORD_TOO_LARGE error but I receive a FDS_ERR_NO_SPACE_IN_FLASH error after writing 290 packets (1160 words). I tried to set the FDS_VIRTUAL_PAGES to 2 and I received the same error after 145 packets. So every virtual page I add, I can receive 145 packets more. I confirmed this by changing the FDS_VIRTUAL_PAGES size to 4 and 5, respectively 435 and 580 packets.

Now im completely lost...

 

Why do I receive a FDS_ERR_NO_SPACE_IN_FLASH while I'm not even close to writing 12.288 bytes (3072 words)?

Why is there an increase of (145 packets * 4 words=)580 words per virtual page?

What is the maximum number of virtual pages I can use?

 

What I want to achieve is to wait for a FDS_ERR_RECORD_TOO_LARGE value, create a new record key and start filling that one.

I hope this all makes sense and someone can enlighten me.

Best regards,

Henrico Brom

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    Hi,

    One of the allocated pages will be tagged as a SWAP page, allowing for garbage collection. During garbage collection, valid records are copied to the SWAP page before the DATA page is erased. The SWAP page is then promoted to a DATA page and the old DATA page is tagged as the new SWAP page. This means that with 3 pages allocated to FDS, you will have the ability to store 2048 words with the default VIRTUAL_PAGE_SIZE.

    The page tag occupies 2 words on each page, leaving 1022 words for data storage. With the 3 words record header, you should be able to store 255 records including a single data word. Can you check/post the output of the flash, to make sure you do not store anything else than the 1 word? You can use nrfjprog for reading flash: 'nrfjprog --memrd 0xFD000 --n 12288' (this reads 3 last pages of nRF52840 flash.

    You should only get the FDS_ERR_RECORD_TOO_LARGE error if you try to store a single record that cannot fit into a single virtual page (default data 1019 words).

    Best regards,
    Jørgen

  • +1 in reply to Jørgen Holmefjord

    Hi Jorgen,

    Thanks for the elaborate answer. Though I found my problem.

    Apparently I was only sending one byte (uint8_t) at a time to the fds instead of a word. After converting my single bytes to words, writing them, reading them and converting them back to bytes, everything worked correctly.

    Also I thought that a word would be written to the same record each time, so expecting eventually a FDS_ERR_RECORD_TOO_LARGE. But this is not the case and a new record is created each time with a unique ID.

    Best regards,

    Henrico 

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  • +1 in reply to Jørgen Holmefjord

    Hi Jorgen,

    Thanks for the elaborate answer. Though I found my problem.

    Apparently I was only sending one byte (uint8_t) at a time to the fds instead of a word. After converting my single bytes to words, writing them, reading them and converting them back to bytes, everything worked correctly.

    Also I thought that a word would be written to the same record each time, so expecting eventually a FDS_ERR_RECORD_TOO_LARGE. But this is not the case and a new record is created each time with a unique ID.

    Best regards,

    Henrico 

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