Using SmartFS


What is SmartFS

SmartFS stands for Sector Mapped Allocation for Really Tiny (SMART) flash. It is a filesystem that has been designed to work primary with small, serial NOR type flash parts that are 1M byte to 16M byte in size (though this is not a limitation). The filesystem operates by segmenting the flash (or flash partition) into “logical sectors” of equal size and then managing them (allocating, mapping, chaining, releasing, etc.) to build files and directories.

SmartFS Code Layering

The system consists of two layers built on top of a standard NuttX MTD driver layer (with it's associated hardware abstraction layer).

The code directly above the NuttX MTD driver is the SMART MTD layer. This interfaces with the MTD (flash driver) layer and handles low-level media operations such as logical sector allocation, freeing and management, erase block management, low-level formatting, wear leveling, etc.

On top of the SMART MTD layer is the Smart Filesystem code. The SmartFS code uses the logical sector services of the SMART MTD layer to provide file and directory level management, such as creating new files, chaining logical sectors together to create files, creating directories and file / directory search and management routines.

Smart FS fs / smartfs / *
SMART MTD drivers / mtd / smart.c
MTD Driver m25px sst25 filmtd etc.
HW Driver spi dev spi dev VFS

Example SmartFS Device Setup

Setting up a device for use with SmartFS is typically done in the config specific source initialization files and would look something like:

int board_app_initialize(uintptr_t arg)
  FAR struct spi_dev_s *spi;
  FAR struct mtd_dev_s *mtd;
  int minor = 0;

  /* Initialize the SPI bus #3 with an M25P FLASH driver */

  spi = stm32_spibus_initialize(3);
  mtd = m25p_initialize(spi);

  /* Initialize SMART MTD to work with M25P FLASH device */

  smart_initialize(minor, mtd, NULL);

Upon successful initialization of the code above, the NuttX Virtual File System (VFS) will contain a new entry called /dev/smart0 to represent the SMART MTD device. Note that this is not a filesystem, but rather a raw block device (which may or may not already be formatted for use with SmartFS). To use the /dev/smart0 as a filesystem, it must be initialized and mounted to the VFS as follows:

nsh> mksmartfs /dev/smart0
nsh> mount -t smartfs /dev/smart0 /mnt

Details of Operation

Pages, Blocks, Sectors and things that FLASH

There are a number of companies that manufacture FLASH parts, and typically they use varying terminology in their data sheets when referring to device geometry. All devices generally have different region sizes for programming, erasing and reading and usually use terms like page, erase block, sector and/or sub-sector. To avoid confusion, NuttX uses the following terminology:

  • Page or Block: The smallest area that a device can program with a single command. This is typically 256 bytes.
  • Erase Block: The smallest area that a device can erase. If multiple erase block sizes are supported (e.g. 4K, 32K and 64K), then the smallest of those sizes, though for larger parts (8M Byte - 32M Byte), sometimes it is better to use the larger erase block size to keep RAM usage to a minimum.
  • Sector: Same as Erase Block.
  • Logical Sector: A size used by a specific filesystem for managing a region independently from the actual device specified blocks / sectors. SmartFS (for example) can use any power of 2 value from 256 through 32768.

Given that each manufacturer and each part has varying geometry sizes, SmartFS uses a Logical Sector whose size is determined when the device is formatted (defaulting to CONFIG_MTD_SMART_SECTOR_SIZE). The SmartFS code then performs all operations using logical sectors and maps physical accesses to the device based on it's reported geometry. This allows filesystem performance tuning (total sectors, minimum allocation size, overhead waste, etc.) independently from the device's erase block size, etc. Each logical sector contains a 10-byte header (5 for MTD layer, 5 for FS layer) for format management.

An example 128K Byte Flash with 32K and 4K Erase Block sizes and 256 byte page read/write sizes:

Bulk Erase Entire Device
Sector Erase 32K 32K 32K 32K
Sector Erase 4K 4K 4K 4K 4K 4K4K 4K
Page read/write 256 256 256 256 256256
Logical Sector (FS) 512 512

In the example above, a SMART MTD logical sector size of 512 bytes was chosen. On the 128K Byte FLASH represented in the table above, this means there would be:

  • 128K / 512 = 256 Logical Sectors Total
  • 4K / 512 = 8 Logical Sectors per Erase Block
  • 512 / 256 = 2 MTD Read/Write Blocks per SMART Logical Sector
  • 10 / 512 = 1.95% Overhead for logical sector headers
  • A maximum of about 250 files / directories supported (considering format overhead)
  • Allocations to files in 512 byte chunks (a zero-length file consumes 512 bytes)

Choosing a SMART Logical sector size of 256 bytes instead would give the following results:

  • 128K / 256 = 512 Logical Sectors Total
  • 4K / 256 = 16 Logical Sectors per Erase Block
  • 256 / 256 = 1 MTD Read/Write Block per SMART Logical Sector
  • 10 / 256 = 3.9% Overhead for logical sector headers
  • A maximum of about 506 files / directories supported (considering format overhead)
  • Allocations to files in 256 byte chunks (a zero-length file consumes 256 bytes)

Checking Your MTD Geometry

Most of the NuttX MTD drivers (in the drivers/mtd directory) have been tested to work with SmartFS / SMART MTD layer and to report block / erase block sizes correctly. However there may be one or more drivers that have not been validated. When configuring SmartFS for the first time using a new / unknown MTD driver, validate the reported geometries are *sane* based on the description above.

Additionally, care must be taken when selecting the SMART MTD logical sector size. The selected size is represented using 3 bits in the logical sector status byte and stored on each sector. This means valid values for the logical sector are:

  • 256, 512
  • 1024, 2048
  • 4096, 8192
  • 16384, 32768

As shown in the table / example above, the logical sector size selection controls the maximum number of files and minimum file size on the volume. There are some limitations to the value selected imposed by the SMART MTD code. The logical sector size must be selected such that:

  • The total number of sectors on the device / partition fits in a 16-bit word. This means 65536 or less total sectors (65536 is supported, though the topmost 2 sectors will never be used).
  • The logical sector size cannot be smaller than the MTD device's block/page size.
  • The logical sector size cannot be larger than the MTD devices's reported erase block size.
  • The total RAM used by the SMART MTD layer is dependent on the logical sector size (when not using Minimize RAM Config option). The selected logical sector size must not create a RAM requirement greater than the available RAM.
  • The number of logical sectors per MTD device erase block cannot be greater than 256, and 4 - 128 is the best choice.
  • If wear-leveling is enabled, then the following condition must be met:
    • Total MTD Erase Blocks / 2 < (Logical Sector Size - 36) * 3

Selecting a logical sector size that creates 256 logical sectors per erase block will create some wasted space on the device. The SMART MTD layer uses a 1-byte variable for the “free sector count” and the “released sector count”. This means it can only track up to 255 logical sectors per erase block. When the sectPerEraseBlk == 256, the last logical sector in each erase block will never be used, thus causing wasted space on the device.

RAM Usage Calculation

Efficient management of the filesystem requires building and maintaining RAM resident status information of the SMART MTD logical sector structure on the physical device. During the 'smart_initialize' function, the code performs a device scan ('smart_scan' routine) to perform this action. Tasks performed by the smart_scan are:

  • Detect if the format sector exists (logical sector zero with “SMRT” tag).
  • Determine the logical sector size that was used to format the device.
  • Locate the root directory logical sectors (logical sector 3-11).
  • Count the number of free sectors on the device and per erase block.
  • Count the number of released sectors on the device and per erase block.


  • Build a map of logical sector to physical sector numbers.


  • Allocate wear-level RAM and read leveling info from the format sector.

The amount of RAM consumed is dependent on the config settings (MINIMIZE_RAM, WEAR_LEVEL, etc.). Rough calculation details are presented for both settings of CONFIG_MTD_SMART_MINIMIZE_RAM:


Item RAM Requirement 1MB / 256 Logical 8MB / 1024 Logical
dev struct 368-380 (approx) 376 376
Logical sector map total_sectors * 2 8192 16384
Erase block free count total erase blocks 16 128
Erase block release count total erase blocks 16 128
Wear status total erase blocks / 2 8 64
MTD sector R/W buffer logical sector size 256 1024
FS sector R/W buffer logical sector size 256 1024
Total 9,176 19,128

On larger volumes, the RAM requirement increases significantly because of the logical sector to physical sector mapping table. To help keep RAM requirement under control, setting the CONFIG_MTD_SMART_MINIMIZE_RAM option eliminates this sector-to-sector map and replaces it with a sector-to-sector cache. The cache size is user defined via the CONFIG_MTD_SMART_SECTOR_CACHE_SIZE option. Using this RAM reduction mode trades off RAM usage for performance.

The cache always contains the format and root-directory logical to physical mapping entries, and then stores additional mappings of recently used logical sectors. When a logical sector is requested that is not contained in the cache, then the MTD device is scanned front-to-back until it's physical location on the device is located. Additionally, if the number of logical sectors per erase block is 16 or less, then the “free count” and “release count” can be packed into a single byte per erase block.


Item RAM Requirement 1MB / 256 Logical 8MB / 1024 Logical
dev struct 368-400 (approx) 392 392
Logical sector cache cache entries * 6 384 384
Free sector bitmap total sectors / 8 512 1024
Erase block free count total erase blocks 16 128
Erase block release count total erase blocks 16 128
Wear status total erase blocks / 2 8 64
MTD sector R/W buffer logical sector size 256 1024
FS sector R/W buffer logical sector size 256 1024
Total 1,832 4,168

Partitions and Mount Points

Creating partitions on the MTD flash / media provides the benefits of physically isolating one filesystem from another, as well as a mechanism for creating multiple mount points within the VFS. There are some pros and cons to consider when deciding to use partitions with SmartFS:

Partitions - PROS

  • Physically separate filesystems … possible errors in one don't affect the other.
  • Smaller partitions on a large device can use smaller logical sector size (good if files are small).
  • Partitions could be used for redundancy.
  • Multiple mount-points within the VFS can be performed if needed.

Partitions - CONS

  • Partition sizes must be understood ahead of time.
  • Directory structure / files cannot span across partitions.
  • Additional overhead is needed for sector management (one erase block + 4 pages/blocks reserved per partition vs. one+4 for entire device).
  • Wear leveling (if enabled) will be limited to operate only within each partition and cannot span multiple partitions globally.

When using partitions with SmartFS, a partition name should be specified during invocation of the 'smart_initialize' routine. Though not a rule, a suggested standard is to provide partition names like 'p1', 'p2', etc. Then the SMART MTD device entries in the /dev directory will take the form:


Prior to the addition of partitions in NuttX, the SmartFS implementation had already implemented a feature which allows multiple VFS mount-points to a single SMART MTD device. This gives the appearance of multiple SmartFS filesystems, though in reality it simply is a single filesystem with multiple logical root-directories. Each of the root-directories on the filesystem will be logically isolated from the others, and after the 'mksmartfs' each will have it's own /dev/smart* entry.

Multi-Root Directory PROS

  • Supports multiple mount points within the NuttX VFS.
  • Each mount-point “appears” to be it's own partition … each directory structure is isolated from the others.
  • Any mount-point / directory structure can occupy as little or as much of the filesystem as needed.
  • Mount-points can be enabled within partitions.
  • Sector management overhead (reserved erase blocks, etc.) are shared by all mount-points.
  • All mount points share RAM structures.
  • Wear leveling is performed evenly over all mount-points.

Multi-Root Directory CONS

  • The logical directories are not physically isolated. Files, directories and individual sectors are all intermixed on the physical device.
  • Any error in the filesystem can effect all mount-points / root directories.

This feature must be enabled specifically using the CONFIG_SMARTFS_MULTI_ROOT_DIRS=y option. Setting this option will cause the SMART MTD /dev entries to be appended with a directory number, such as 'd1', 'd2', etc. Prior to creating a SmartFS filesystem on the raw device, a single entry will be identified, such as:

/dev/smart1p1d1    (device with partitions and multi-root directories)

Then after executing the 'mksmartfs' command, additional entries will appear (each of which can be mounted to a different VFS location):

nsh> ls /dev
nsh> mksmartfs /dev/smart0d1 3
nsh> ls /dev
nsh> mount -t smartfs /dev/smart0d1 /data
nsh> mount -t smartfs /dev/smart0d2 /apps
nsh> mount -t smartfs /dev/smart0d3 /recover

The ProcFS Interface

When the PROCFS interface is enabled, each mounted SmartFS device will appear under:


The pseudo files reported for each entry will depend on the configured options. Entries that can currently appear are:

Entry CONFIG_MTD_SMART_* Meaning
status Report volume status including geometry
debuglevel Write ASCII '0' - '2' to set debug print level
erasemap WEAR_LEVEL=y Report map (A-N) of erase block erasures
mem ALLOC_DEBUG=y Print report of all SMART MTD memory allocs

Example procfs usage:

nsh> mount -t smartfs /dev/smart0 /mnt
nsh> mount -t procfs /proc
nsh> cat /proc/fs/smartfs/smart0/status

Format version:    1
Name Len:          16
Total Sectors:     4096
Sector Size:       256
Format Sector:     0
Dir Sector:        256
Free Sectors:      4078
Released Sectors:  0
Unused Sectors:    0
Block Erases:      0
Sectors Per Block: 256
Sector Utilization:100%
Uneven Wear Count: 0

nsh> cat /proc/fs/smartfs/smart0/erasemap


When Things Don't Work

The SmartFS code and MTD layer have been pretty well tested and used in production products. If things aren't working for you, there are a couple of places to start debugging first.

Check FLASH MTD Driver

Ensure the geometry of the FLASH MTD driver is following the NuttX standard for block / erase block geometry sizes. Most of them do, but via simple inspection of the code (as of version 7.16, July 12, 1016), the drivers that are likely to have improper Geometry reporting (and thus incompatible with SmartFS) are:

  • s25fl1.c
  • sst39vf.c

When configuring to use SmartFS, check the reported geometry of the MTD driver you are using. If the geo.blocksize is reported to be the same as the geo.sectorsize, then there is likely an issue with the MTD driver implementation. But it is likely to be a reporting problem of the geo.blocksize that is incorrect AND *possibly* the starting address calculations in the _bwrite / _bread routines may be incorrect. These are BLOCK read / write operations and calculations need to be performed using the block size (typically 256), not the sector size (4K, 32K, etc.).

Check CONFIG Options

Double check all of the CONFIG options for both the MTD driver and the SMART MTD layer:

  • CONFIG_MTD_SMART_SECTOR_SIZE: Ensure it's not smaller than the block size or larger than erase block size.
  • CONFIG_MTD_BYTE_WRITE: If enabled, ensure the FLASH actually supports this mode (single byte programming).
  • CONFIG_MTD_XXX_SECTOR512: Ensure this is not set. This should only be use with FAT volumes.
  • CONFIG_MTD_XXX_MANUFACTURER: Double check this with the data sheet / MTD driver code
  • CONFIG_MTD_XXX_MEMORY_TYPE: Double check this with the data sheet

Check Writability to the Part

Ensure the FLASH can be written successfully. If the WP pin is pull active (i.e. the part is write protected), then the SMART MTD layer will not be able to write any data. Additionally validate there are no individual protected sectors on the device.