Maximum settings partition size estimation

Hi there,

I need help finalizing the size of my product's settings partition. I only store Bluetooth bonds on the settings partition: 20 bonds to other Nordic devices via the Nordic UART service and 10 bonds to BLE HID devices. These connections must be securely encrypted.

According to what I see, the following settings keys are stored:
- bt/sc/0123456789abc 4 bytes, stored for one device
- bt/cf/0123456789abc - 2 bytes, stored for one device
- bt/keys/0123456789abc - 80 bytes, stored for all devices
- bt/ccc/0123456789abc - 12-32 bytes, stored for all devices

The reason I can't estimate the maximum storage size for my needs is because of the following:
- Some keys are present for all devices I tested, while others are only for one. I don't know which keys are stored in which cases.
- Some keys vary in size, and I don't know their minimum and maximum size.
- There may be additional keys that are used on select occasions, which I don't know.
- I don't know the storage overhead of NVS.
- Can NVS be fragmented? If so, it should be accounted for.

Thanks in advance!
- Laci

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+1 Vidar Berg 2 months ago

Hello,

How much flash is used by settings is very application depended (number of setting entries and how frequent they are updated). I recommend using the nvs_calc_free_space() to help profile the flash usage for your application. Flash wear should also be considered: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/services/storage/nvs/nvs.html#flash-wear

You can use this function to retreive the NVS instance used by the settings module: https://github.com/nrfconnect/sdk-zephyr/commit/ccf8c547d090ad3f76dbfa64744f32333658dcdb

Best regards,

Vidar
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0 mlac 1 month ago in reply to Vidar Berg

Hi Vidar,

I've included the following in my main():

	void *storage;
    int rc = settings_storage_get(&storage);
    ssize_t size = nvs_calc_free_space((struct nvs_fs *)storage);
    printk("Free space: %d\n", size);

According to the printed result, the free space doesn't exceed 4K, even though I've allocated 8K for storage.

my-board.dtsi:

&flash0 {
    partitions {
        compatible = "fixed-partitions";
        #address-cells = <1>;
        #size-cells = <1>;

        boot_partition: partition@0 {
            label = "mcuboot";
            reg = <0x0 0x10000>;
        };
        slot0_partition: partition@10000 {
            label = "image-0";
            reg = <0x10000 0xdd000>;
        };
        hardware_config_partition: partition@ed000 {
            label = "hardware-config";
            reg = <0xed000 0x1000>;
        };
        user_config_partition: partition@ee000 {
            label = "user-config";
            reg = <0xee000 0x10000>;
        };
        storage_partition: partition@fe000 {
            label = "storage";
            reg = <0xfe000 0x2000>;
        };
    };
};

pm_static.yml:

app:
  address: 0x10200
  end_address: 0xfe000
  region: flash_primary
  size: 0xede00
mcuboot:
  address: 0x0
  end_address: 0x10000
  placement:
    before:
    - mcuboot_primary
  region: flash_primary
  size: 0x10000
mcuboot_pad:
  address: 0x10000
  end_address: 0x10200
  placement:
    align:
      start: 0x1000
    before:
    - mcuboot_primary_app
  region: flash_primary
  size: 0x200
mcuboot_primary:
  address: 0x10000
  end_address: 0xfe000
  orig_span: &id001
  - app
  - mcuboot_pad
  region: flash_primary
  size: 0xee000
  span: *id001
mcuboot_primary_app:
  address: 0x10200
  end_address: 0xfe000
  orig_span: &id002
  - app
  region: flash_primary
  size: 0xede00
  span: *id002
hardware_config_partition:
  address: 0xed000
  end_address: 0xee000
  size: 0x1000
user_config_partition:
  address: 0xee000
  end_address: 0xfe000
  size: 0x10000
settings_storage:
  address: 0xfe000
  end_address: 0x100000
  placement:
    align:
      start: 0x1000
    before:
    - end
  region: flash_primary
  size: 0x2000
sram_primary:
  address: 0x20000000
  end_address: 0x20040000
  region: sram_primary
  size: 0x40000

Thanks in advance for your help!

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