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 a loop is detected NVS returns that there is no more space available.

For NVS the file system is declared as:

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

where

NVS_FLASH_DEVICE is a reference to the flash device that will be used. The device needs to be operational.
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.

Flash write block size migration

It is possible that during a DFU process, the flash driver used by the NVS changes the supported minimal write block size. The NVS in-flash image will stay compatible unless the physical ATE size changes. Especially, migration between 1,2,4,8-bytes write block sizes is allowed.

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.

Public Members

off_t offset: File system offset in flash.

uint32_t ate_wra

Allocation table entry write address.

Addresses are stored as uint32_t:

high 2 bytes correspond to the sector
low 2 bytes are the offset in the sector

uint32_t data_wra: Data write address.

uint16_t sector_size: File system is split into sectors, each sector must be multiple of erase-block-size.

uint16_t sector_count: Number of sectors in the file system.

bool ready: Flag indicating if the file system is initialized.

struct k_mutex nvs_lock: Mutex.

const struct device *flash_device: Flash device runtime structure.

const struct flash_parameters *flash_parameters: Flash memory parameters structure.

group nvs_high_level_api

Non-volatile Storage APIs.

Functions

int nvs_mount(struct nvs_fs *fs)

Mount an NVS file system onto the flash device specified in fs.

Parameters:

fs – Pointer to file system

Return values:

0 – Success
-ERRNO – errno code if error

int nvs_clear(struct nvs_fs *fs)

Clear the NVS file system from flash.

Parameters:

fs – Pointer to file system

Return values:

0 – Success
-ERRNO – errno code if error

ssize_t nvs_write(struct nvs_fs *fs, uint16_t id, const void *data, size_t len)

Write an entry to the file system.

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

Returns:

Number of bytes written. On success, it will be equal to the number of bytes requested to be written. When a rewrite of the same data already stored is attempted, nothing is written to flash, thus 0 is returned. On error, returns negative value of errno.h defined error codes.

int nvs_delete(struct nvs_fs *fs, uint16_t id)

Delete an entry from the file system.

Parameters:

fs – Pointer to file system
id – Id of the entry to be deleted

Return values:

0 – Success
-ERRNO – errno code if error

ssize_t nvs_read(struct nvs_fs *fs, uint16_t id, void *data, size_t len)

Read an entry from the file system.

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

Returns:

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 negative value of errno.h defined error codes.

ssize_t nvs_read_hist(struct nvs_fs *fs, uint16_t id, void *data, size_t len, uint16_t cnt)

Read a history entry from the file system.

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 …

Returns:

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 negative value of errno.h defined error codes.

ssize_t nvs_calc_free_space(struct nvs_fs *fs)

Calculate the available free space in the file system.

Parameters:

fs – Pointer to file system

Returns:

Number of bytes free. On success, it will be equal to the number of bytes that can still be written to the file system. Calculating the free space is a time consuming operation, especially on spi flash. On error, returns negative value of errno.h defined error codes.