General Steps

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This document summarizes the general implementation process of Spiffs in ESP-IDF, covering its basic description, initialization steps, and related execution process.

By mastering the content of this document, developers can quickly understand the key logic of Spiffs and provide a unified reference for subsequent example learning.

Spiffs Overview

Spiffs (SPI Flash File System) is a lightweight file system designed for embedded devices, running on SPI interface Flash memory. With Spiffs, Flash space can be used as a small file system, allowing applications to store and retrieve data like operating files, without having to directly handle the underlying sector erasure, address management, and data verification.

In embedded systems, although data can be saved by directly reading and writing Flash, this method requires developers to manage erasure, offset, and life control on their own, which is cumbersome and prone to errors. The advantage of Spiffs is that it provides a unified abstract interface for Flash, hiding the complex underlying storage logic, allowing developers to focus on storage needs in units of files.

The application value of Spiffs is mainly reflected in the following aspects:

  • Simplify development: Access data through the file interface, rather than directly operating sectors.

  • Improve maintainability: Data and programs are separated, file content can be updated separately, without having to recompile the entire firmware.

  • Save resources: As a file system optimized for embedded systems, it occupies very little space and resources.

  • Persistent storage: Data can still be saved after power loss, and it has a basic wear leveling mechanism, which can extend the life of Flash, but it is not suitable for frequently writing large data.

Therefore, Spiffs is often used in embedded applications to store configuration files, web resources, certificates, and some small data that does not need to be frequently modified. It is an important tool connecting underlying storage and application logic.

Flash Storage and Partition Table

In Flash, data can only be erased in units of sectors, and the corresponding sectors must be erased before writing. The emergence of Spiffs is to shield these underlying restrictions, wrap a continuous Flash space into a file system, and allow users to access and store in units of files.

In the ESP-IDF framework, the use of Flash needs to be allocated through the partition table. As a file system, Spiffs also needs to define a dedicated storage area in the partition table. Only when space is reserved for Spiffs in the partition table can the system successfully mount and use this file system.

Spiffs Space Division

In embedded systems, the entire Flash storage space often needs to accommodate multiple functional areas such as program firmware, NVS storage, OTA partition, and file system. To avoid interference with each other, ESP-IDF divides Flash through the partition table. As a file system, Spiffs needs to reserve a continuous storage area in the partition table.

The core of Spiffs space division is to reserve an area for it in the entire Flash space. The system identifies and mounts this area through the partition table, and Spiffs organizes file data and meta-information in it. Reasonable division not only ensures functionality but also improves storage efficiency and system reliability.

Significance of Partition Table Division

  • Each partition table entry corresponds to a segment of space in Flash, recording its start address, size, purpose, and name.

  • Spiffs partitions are generally marked as data type, with a subtype of spiffs.

  • The size of the partition is determined by the developer, which determines the maximum available storage space in the Spiffs file system.

Internal Structure of Space

Inside the Spiffs partition, the space will be further divided according to the needs of the file system:

  • File data area: Used to store the actual file content.

  • Metadata area: Used to record file name, size, status, and other information.

  • Free Area: Unused storage space for new files or file extensions.

  • Erase Block Management: Due to the erase limitations of Flash, Spiffs internally implements a wear leveling mechanism that dynamically allocates and recycles blocks to extend the life of the Flash.

Trade-off of Space Size

  • If the partition is too small, only a very small number of files can be stored, which is not suitable for storing web resources or multiple configuration files.

  • If the partition is too large, it will squeeze the space of the program firmware or other data partitions.

  • During development, allocation should be made reasonably according to actual application requirements, such as storing configuration files of several hundred KB or web resources of several MB.

General Steps of Spiffs

Mounting and Initialization

Spiffs must be mounted before use to establish a file system environment. During the mounting process, the storage partition will be mapped to a specified virtual path, allowing the application program to access this area like operating a regular file. During the initialization process, the system will check the integrity of the file system, determine whether to format the partition, and configure related parameters to ensure that subsequent read and write operations can be performed safely and reliably.

  • Configure related parameters through the structure esp_vfs_spiffs_conf_t, including the mounting path, the maximum number of files that can be opened, whether to automatically format when mounting fails, etc. For specific structure members, refer to SPIFFS File System <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html#_CPPv421esp_vfs_spiffs_conf_t>.

  • Call esp_vfs_spiffs_register() to complete initialization and mounting. This function will try to mount the Spiffs partition. If the mounting fails and automatic formatting is set, it will format the partition and finally return the mounting result. For related usage and parameter description, refer to SPIFFS File System <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html#_CPPv423esp_vfs_spiffs_registerPK21esp_vfs_spiffs_conf_t>.

Checksum and Partition Information Check

To ensure the reliability of the file system, it is necessary to check the checksum and information of Spiffs. This step can detect potential data corruption, power failure anomalies, or inconsistent states caused by write failures, and provide repair or cleaning mechanisms. By obtaining information about the total capacity of the partition and the used space, developers can also be assisted in storage management and capacity monitoring.

  • Call esp_spiffs_check() to check the consistency of the file system. It is usually executed in cases where power failure, write anomalies, or partition damage may occur, to ensure the integrity of file system data. For related usage and parameter description, refer to SPIFFS File System <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html#_CPPv416esp_spiffs_checkPKc>. The CONFIG_EXAMPLE_SPIFFS_CHECK_ON_START macro needs to be enabled during execution.

  • Call esp_spiffs_info() to get partition size information, that is, the total capacity of the partition and the information of the used capacity, to judge the current status of the file system. For related usage and parameter description, refer to SPIFFS File System <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/storage/spiffs.html#_CPPv415esp_spiffs_infoPKcP6size_tP6size_t>.

  • If partition information retrieval fails, you can call esp_spiffs_format() to format the partition, ensuring the file system is available. For related usage and parameter descriptions, please refer to SPIFFS File System.

  • Check the consistency of partition size. If the recorded used space in the file system exceeds the total capacity, it indicates possible data anomalies or file system damage. You need to call esp_spiffs_check() again for repair or cleanup of unreferenced data blocks. For related usage and parameter descriptions, please refer to SPIFFS File System.