Flash layout

Even though file system is stored on the same flash chip as the program, programming new sketch will not modify file system contents. This allows to use file system to store sketch data, configuration files, or content for Web server.

The following diagram illustrates flash layout used in Arduino environment:

^              ^       ^               ^     ^
Sketch    OTA update   File system   EEPROM  WiFi config (SDK)

File system size depends on the flash chip size. Depending on the board which is selected in IDE, you have the following options for flash size:

Board Flash chip size, bytes File system size, bytes
Generic module 512k 64k, 128k
Generic module 1M 64k, 128k, 256k, 512k
Generic module 2M 1M
Generic module 4M 1M, 2M, 3M
Adafruit HUZZAH 4M 1M, 2M, 3M
ESPresso Lite 1.0 4M 1M, 2M, 3M
ESPresso Lite 2.0 4M 1M, 2M, 3M
NodeMCU 0.9 4M 1M, 2M, 3M
NodeMCU 1.0 4M 1M, 2M, 3M
Olimex MOD-WIFI-ESP8266(-DEV) 2M 1M
SparkFun Thing 512k 64k
SweetPea ESP-210 4M 1M, 2M, 3M
WeMos D1 R1, R2 & mini 4M 1M, 2M, 3M
ESPDuino 4M 1M, 2M, 3M
WiFiduino 4M 1M, 2M, 3M

Note: to use any of file system functions in the sketch, add the following include to the sketch:

#include "FS.h"

SPIFFS and LittleFS

There are two filesystems for utilizing the onboard flash on the ESP8266: SPIFFS and LittleFS.

SPIFFS is the original filesystem and is ideal for space and RAM constrained applications that utilize many small files and care about static and dynamic wear levelling and don’t need true directory support. Filesystem overhead on the flash is minimal as well.

LittleFS is recently added and focuses on higher performance and directory support, but has higher filesystem and per-file overhead (4K minimum vs. SPIFFS’ 256 byte minimum file allocation unit).

They share a compatible API but have incompatible on-flash implementations, so it is important to choose one or the per project as attempting to mount a SPIFFS volume under LittleFS may result in a format operation and definitely will not preserve any files, and vice-versa.

The actual File and Dir objects returned from either filesystem behave in the same manner and documentation is applicable to both. To convert most applications from SPIFFS to LittleFS simply requires changing the SPIFFS.begin() to LittleFS.begin() and to with the rest of the code remaining untouched.

SPIFFS file system limitations

The SPIFFS implementation for ESP8266 had to accomodate the constraints of the chip, among which its limited RAM. SPIFFS was selected because it is designed for small systems, but that comes at the cost of some simplifications and limitations.

First, behind the scenes, SPIFFS does not support directories, it just stores a “flat” list of files. But contrary to traditional filesystems, the slash character '/' is allowed in filenames, so the functions that deal with directory listing (e.g. openDir("/website")) basically just filter the filenames and keep the ones that start with the requested prefix (/website/). Practically speaking, that makes little difference though.

Second, there is a limit of 32 chars in total for filenames. One '\0' char is reserved for C string termination, so that leaves us with 31 usable characters.

Combined, that means it is advised to keep filenames short and not use deeply nested directories, as the full path of each file (including directories, '/' characters, base name, dot and extension) has to be 31 chars at a maximum. For example, the filename /website/images/bird_thumbnail.jpg is 34 chars and will cause some problems if used, for example in exists() or in case another file starts with the same first 31 characters.

Warning: That limit is easily reached and if ignored, problems might go unnoticed because no error message will appear at compilation nor runtime.

For more details on the internals of SPIFFS implementation, see the SPIFFS readme file.

LittleFS file system limitations

The LittleFS implementation for the ESP8266 supports filenames of up to 31 characters + terminating zero (i.e. char filename[32]), and as many subdirectories as space permits.

Filenames are assumed to be in the root directory if no initial “/” is present.

Opening files in subdirectories requires specifying the complete path to the file (i.e. open("/sub/dir/file.txt");). Subdirectories are automatically created when you attempt to create a file in a subdirectory, and when the last file in a subdirectory is removed the subdirectory itself is automatically deleted. This is because there was no mkdir() method in the existing SPIFFS filesystem.

Unlike SPIFFS, the actual file descriptors are allocated as requested by the application, so in low memory conditions you may not be able to open new files. Conversely, this also means that only file descriptors used will actually take space on the heap.

Because there are directories, the openDir method behaves differently than SPIFFS. Whereas SPIFFS will return files in “subdirectories” when you traverse a Dir::next() (because they really aren’t subdirs but simply files with “/”s in their names), LittleFS will only return files in the specific subdirectory. This mimics the POSIX behavior for directory traversal most C programmers are used to.

Uploading files to file system

ESP8266FS is a tool which integrates into the Arduino IDE. It adds a menu item to Tools menu for uploading the contents of sketch data directory into ESP8266 flash file system.

Warning: Due to the move from the obsolete esptool-ck.exe to the supported upload tool, upgraders from pre 2.5.1 will need to update the ESP8266FS tool referenced below to 0.4.0 or later. Prior versions will fail with a “esptool not found” error because they don’t know how to use

  • Download the tool:
  • In your Arduino sketchbook directory, create tools directory if it doesn’t exist yet
  • Unpack the tool into tools directory (the path will look like <home_dir>/Arduino/tools/ESP8266FS/tool/esp8266fs.jar) If upgrading, overwrite the existing JAR file with the newer version.
  • Restart Arduino IDE
  • Open a sketch (or create a new one and save it)
  • Go to sketch directory (choose Sketch > Show Sketch Folder)
  • Create a directory named data and any files you want in the file system there
  • Make sure you have selected a board, port, and closed Serial Monitor
  • Select Tools > ESP8266 Sketch Data Upload. This should start uploading the files into ESP8266 flash file system. When done, IDE status bar will display SPIFFS Image Uploaded message.

ESP8266LittleFS is the equivalent tool for LittleFS.

File system object (SPIFFS/LittleFS)


SPIFFSConfig cfg;

This method allows you to configure the parameters of a filesystem before mounting. All filesystems have their own *Config (i.e. SDFSConfig or SPIFFSConfig with their custom set of options. All filesystems allow explicitly enabling/disabling formatting when mounts fail. If you do not call this setConfig method before perforing begin(), you will get the filesystem’s default behavior and configuration. By default, SPIFFS will autoformat the filesystem if it cannot mount it, while SDFS will not.


or LittleFS.begin()

This method mounts file system. It must be called before any other FS APIs are used. Returns true if file system was mounted successfully, false otherwise. With no options it will format SPIFFS if it is unable to mount it on the first try.

Note that both methods will automatically format the filesystem if one is not detected. This means that if you attempt a SPIFFS.begin() on a LittleFS filesystem you will lose all data on that filesystem, and vice-versa.


or LittleFS.end()

This method unmounts the file system. Use this method before updating the file system using OTA.


or LittleFS.format()

Formats the file system. May be called either before or after calling begin. Returns true if formatting was successful.

open, mode)
or, mode)

Opens a file. path should be an absolute path starting with a slash (e.g. /dir/filename.txt). mode is a string specifying access mode. It can be one of “r”, “w”, “a”, “r+”, “w+”, “a+”. Meaning of these modes is the same as for fopen C function.

r      Open text file for reading.  The stream is positioned at the
       beginning of the file.

r+     Open for reading and writing.  The stream is positioned at the
       beginning of the file.

w      Truncate file to zero length or create text file for writing.
       The stream is positioned at the beginning of the file.

w+     Open for reading and writing.  The file is created if it does
       not exist, otherwise it is truncated.  The stream is
       positioned at the beginning of the file.

a      Open for appending (writing at end of file).  The file is
       created if it does not exist.  The stream is positioned at the
       end of the file.

a+     Open for reading and appending (writing at end of file).  The
       file is created if it does not exist.  The initial file
       position for reading is at the beginning of the file, but
       output is always appended to the end of the file.

Returns File object. To check whether the file was opened successfully, use the boolean operator.

File f ="/f.txt", "w");
if (!f) {
    Serial.println("file open failed");


or LittleFS.exists(path)

Returns true if a file with given path exists, false otherwise.



Returns true if the directory creation succeeded, false otherwise.



Returns true if the directory was successfully removed, false otherwise.


or LittleFS.openDir(path)

Opens a directory given its absolute path. Returns a Dir object. Please note the previous discussion on the difference in behavior between LittleFS and SPIFFS for this call.


or LittleFS.remove(path)

Deletes the file given its absolute path. Returns true if file was deleted successfully.


SPIFFS.rename(pathFrom, pathTo)
or LittleFS.rename(pathFrom, pathTo)

Renames file from pathFrom to pathTo. Paths must be absolute. Returns true if file was renamed successfully.


FSInfo fs_info;;

Fills FSInfo structure with information about the file system. Returns true is successful, false otherwise.

Filesystem information structure

struct FSInfo {
    size_t totalBytes;
    size_t usedBytes;
    size_t blockSize;
    size_t pageSize;
    size_t maxOpenFiles;
    size_t maxPathLength;

This is the structure which may be filled using FS::info method. - totalBytes — total size of useful data on the file system - usedBytes — number of bytes used by files - blockSize — filesystem block size - pageSize — filesystem logical page size - maxOpenFiles — max number of files which may be open simultaneously - maxPathLength — max file name length (including one byte for zero termination)



Only implemented in SPIFFS. Performs a quick garbage collection operation on SPIFFS, possibly making writes perform faster/better in the future. On very full or very fragmented filesystems, using this call can avoid or reduce issues where SPIFFS reports free space but is unable to write additional data to a file. See this discussion <> for more info.



Only implemented in SPIFFS. Performs an in-depth check of the filesystem metadata and correct what is repairable. Not normally needed, and not guaranteed to actually fix anything should there be corruption.

Directory object (Dir)

The purpose of Dir object is to iterate over files inside a directory. It provides multiple access methods.

The following example shows how it should be used:

Dir dir = SPIFFS.openDir("/data");
// or Dir dir = LittleFS.openDir("/data");
while ( {
    if(dir.fileSize()) {
        File f = dir.openFile("r");


Returns the name of the current file pointed to by the internal iterator.


Returns the size of the current file pointed to by the internal iterator.


Returns true if the current file pointed to by the internal iterator is a File.


Returns true if the current file pointed to by the internal iterator is a Directory.


This method takes mode argument which has the same meaning as for SPIFFS/ function.


Resets the internal pointer to the start of the directory.

File object

SPIFFS/ and dir.openFile() functions return a File object. This object supports all the functions of Stream, so you can use readBytes, findUntil, parseInt, println, and all other Stream methods.

There are also some functions which are specific to File object.

seek, mode)

This function behaves like fseek C function. Depending on the value of mode, it moves current position in a file as follows:

  • if mode is SeekSet, position is set to offset bytes from the beginning.
  • if mode is SeekCur, current position is moved by offset bytes.
  • if mode is SeekEnd, position is set to offset bytes from the end of the file.

Returns true if position was set successfully.



Returns the current position inside the file, in bytes.



Returns file size, in bytes.


String name =;

Returns short (no-path) file name, as const char*. Convert it to String for storage.


// Filesystem:
//   testdir/
//           file1
Dir d = LittleFS.openDir("testdir/");
File f = d.openFile("r");
// == "file1", f.fullName() == "testdir/file1"

Returns the full path file name as a const char*.


bool amIAFile = file.isFile();

Returns true if this File points to a real file.


bool amIADir = file.isDir();

Returns true if this File points to a directory (used for emulation of the SD.* interfaces with the openNextFile method).



Close the file. No other operations should be performed on File object after close function was called.