Non-Volatile Storage (NVS)

Elements, represented as id-data pairs, are stored in flash using a FIFO-managed circular buffer. The flash area is divided into sectors. Elements are appended to a sector until storage space in the sector is exhausted. Then a new sector in the flash area is prepared for use (erased). Before erasing the sector it is checked that identifier - data pairs exist in the sectors in use, if not the id-data pair is copied.

The id is a 16-bit unsigned number. NVS ensures that for each used id there is at least one id-data pair stored in flash at all time.

NVS allows storage of binary blobs, strings, integers, longs, and any combination of these.

Each element is stored in flash as metadata (8 byte) and data. The metadata is written in a table starting from the end of a nvs sector, the data is written one after the other from the start of the sector. The metadata consists of: id, data offset in sector, data length, part (unused) and a crc.

A write of data to nvs always starts with writing the data, followed by a write of the metadata. Data that is written in flash without metadata is ignored during initialization.

During initialization NVS will verify the data stored in flash, if it encounters an error it will ignore any data with missing/incorrect metadata.

NVS checks the id-data pair before writing data to flash. If the id-data pair is unchanged no write to flash is performed.

To protect the flash area against frequent erases it is important that there is sufficient free space. NVS has a protection mechanism to avoid getting in a endless loop of flash page erases when there is limited free space. When such an endless loop is detected NVS is placed in a locked state and becomes a read-only file system.

For NVS the file system is declared as:

static struct nvs_fs fs = {
.sector_size = NVS_SECTOR_SIZE,
.sector_count = NVS_SECTOR_COUNT,
.offset = NVS_STORAGE_OFFSET,
};

where

  • NVS_SECTOR_SIZE is the sector size, it has to be a multiple of the flash erase page size and a power of 2.
  • NVS_SECTOR_COUNT is the number of sectors, it is at least 2, one sector is always kept empty to allow copying of existing data.
  • NVS_STORAGE_OFFSET is the offset of the storage area in flash.

Flash wear

When writing data to flash a study of the flash wear is important. Flash has a limited life which is determined by the number of times flash can be erased. Flash is erased one page at a time and the pagesize is determined by the hardware. As an example a nRF51822 device has a pagesize of 1024 bytes and each page can be erased about 20,000 times.

Calculating expected device lifetime

Suppose we use a 4 bytes state variable that is changed every minute and needs to be restored after reboot. NVS has been defined with a sector_size equal to the pagesize (1024 bytes) and 2 sectors have been defined.

Each write of the state variable requires 12 bytes of flash storage: 8 bytes for the metadata and 4 bytes for the data. When storing the data the first sector will be full after 1024/12 = 85.33 minutes. After another 85.33 minutes, the second sector is full. When this happens, because we’re using only two sectors, the first sector will be used for storage and will be erased after 171 minutes of system time. With the expected device life of 20,000 writes, with two sectors writing every 171 minutes, the device should last about 171 * 20,000 minutes, or about 6.5 years.

More generally then, with

  • NS as the number of storage requests per minute,
  • DS as the data size in bytes,
  • SECTOR_SIZE in bytes, and
  • PAGE_ERASES as the number of times the page can be erased,

the expected device life (in minutes) can be calculated as:

SECTOR_COUNT * SECTOR_SIZE * PAGE_ERASES / (NS * (DS+8)) minutes

From this formula it is also clear what to do in case the expected life is too short: increase SECTOR_COUNT or SECTOR_SIZE.

Sample

A sample of how NVS can be used is supplied in samples/subsys/nvs.

Troubleshooting

MPU fault while using NVS, or -ETIMEDOUT error returned
NVS can use the internal flash of the SoC. While the MPU is enabled, the flash driver requires MPU RWX access to flash memory, configured using CONFIG_MPU_ALLOW_FLASH_WRITE. If this option is disabled, the NVS application will get an MPU fault if it references the internal SoC flash and it’s the only thread running. In a multi-threaded application, another thread might intercept the fault and the NVS API will return an -ETIMEDOUT error.

API Reference

The NVS subsystem APIs are provided by nvs.h:

group nvs_data_structures

Non-volatile Storage Data Structures.

struct nvs_fs
#include <nvs.h>

Non-volatile Storage File system structure.

Parameters
  • offset: File system offset in flash
  • ate_wra: Allocation table entry write address. Addresses are stored as u32_t: high 2 bytes are sector, low 2 bytes are offset in sector,
  • data_wra: Data write address.
  • sector_size: File system is divided into sectors each sector should be multiple of pagesize
  • sector_count: Amount of sectors in the file systems
  • write_block_size: Alignment size
  • locked: State of the filesystem, locked = true means the filesystem is read only
  • nvs_lock: Mutex
  • flash_device: Flash Device

group nvs_high_level_api

Non-volatile Storage APIs.

Functions

int nvs_init(struct nvs_fs *fs, const char *dev_name)

nvs_init

Initializes a NVS file system in flash.

Parameters
  • fs: Pointer to file system
  • dev_name: Pointer to flash device name
Return Value
  • 0: Success
  • -ERRNO: errno code if error

int nvs_reinit(struct nvs_fs *fs)

nvs_reinit

Reinitializes a NVS file system in flash, if the filesystem is locked this will erase the filesystem.

Parameters
  • fs: Pointer to file system
Return Value
  • 0: Success
  • -ERRNO: errno code if error

int nvs_clear(struct nvs_fs *fs)

nvs_clear

Clears the NVS file system from flash.

Parameters
  • fs: Pointer to file system
Return Value
  • 0: Success
  • -ERRNO: errno code if error

ssize_t nvs_write(struct nvs_fs *fs, u16_t id, const void *data, size_t len)

nvs_write

Write an entry to the file system.

Return
Number of bytes written. On success, it will be equal to the number of bytes requested to be written. On error returns -ERRNO code.
Parameters
  • fs: Pointer to file system
  • id: Id of the entry to be written
  • data: Pointer to the data to be written
  • len: Number of bytes to be written

int nvs_delete(struct nvs_fs *fs, u16_t id)

nvs_delete

Delete an entry from the file system

Parameters
  • fs: Pointer to file system
  • id: Id of the entry to be deleted
Return Value
  • 0: Success
  • -ERRNO: errno code if error

ssize_t nvs_read(struct nvs_fs *fs, u16_t id, void *data, size_t len)

nvs_read

Read an entry from the file system.

Return
Number of bytes read. On success, it will be equal to the number of bytes requested to be read. When the return value is larger than the number of bytes requested to read this indicates not all bytes were read, and more data is available. On error returns -ERRNO code.
Parameters
  • fs: Pointer to file system
  • id: Id of the entry to be read
  • data: Pointer to data buffer
  • len: Number of bytes to be read

ssize_t nvs_read_hist(struct nvs_fs *fs, u16_t id, void *data, size_t len, u16_t cnt)

nvs_read_hist

Read a history entry from the file system.

Return
Number of bytes read. On success, it will be equal to the number of bytes requested to be read. When the return value is larger than the number of bytes requested to read this indicates not all bytes were read, and more data is available. On error returns -ERRNO code.
Parameters
  • fs: Pointer to file system
  • id: Id of the entry to be read
  • data: Pointer to data buffer
  • len: Number of bytes to be read
  • cnt: History counter: 0: latest entry, 1:one before latest …