Version 5 (modified by ChrisJohns, on Feb 23, 2010 at 7:10:43 AM) (diff)


RTEMS File System

The RTEMS File System or RFS provides a fully featured file system that is fast and compact. The RFS interfaces to the Block Devices API? and therefore supports all the block devices RTEMS has. It also interfaces to the RTEMS libc file system support layer. This means the POSIX interface to files in RTEMS is supported by the RFS file system.

RFS Structure

The RFS file system consists of superblock at the start followed by a series of groups. A group is a group of blocks and exists to keep the bitmaps in a group to a file block size plus to also help localise blocks and inodes.


! style="background:#efefef;" | Superblock

! align="center" | Group

! align="center" | Group |} </blockquote>

A group is broken up into the block allocation bitmap the inode allocation bitmap, the inodes, and the data blocks.


! align="center' | Block Bitmap

! align="center" | Inode Bitmap

! align="center" | Inodes

! align="center" | Data Blocks |} </blockquote>

The block bitmap contains a single bit for every block in a group. The default format is to allocate the size of a group based on the number of bits that fit into a single block. For example a block of 1024 bytes has 8,192 bits therefore a group can have 8192 blocks including the block bitmap block. The inode bitmap has a bit for ever inode in the group. An inode is the information about a node on the disk. A node is the data that links the elements of the disk together to create directories, files, and nodes as well as hold times and flags. The number of inodes in a group is a format configuration parameter. The default is use 1% of the disk's blocks for inode data. Again with a 1024 byte block size there are 18 inodes per block and 1448 inodes per group. Inodes do not span block boundaries.


An inode can be a device node (block or character), directory, FIFO, regular file, link, or socket. Current RTEMS does not support FIFO or socket nodes in its file systems so they are ignored. Inodes are numbered from 1 so there is no inode number 0. The root inode is always 1 and therefore the first inode of the first group and its mode is directory. This directory can contain any number of inodes including directories. Files can be link using hard or symbolic links. Directories cannot be linked therefore avoiding the creation of a directed acyclic graph or DAG. An inode can have a map of blocks associated to it. The use of the data depends on the mode of the inode. If the inode is a directory the data in the block map is the contents of the directory. If a regular file the block map contains the file's data and if a symbolic link the data is the link path. The block map has three levels. The first is a series of block numbers held in the inode. The current value is 5 blocks. When the block map is less than or equal to 5 blocks the mapping is called direct as the block numbers map directly to data blocks. If there are more than 5 blocks the 5 inode slots point to blocks that hold the data blocks. This called singly indirect. The number of blocks that can be addressed with a single indirect map depends on the block size. For a block size of 1024 bytes and a 32bit block number a block holds 256 blocks, and with 5 indirect blockes addressable from the inode this is a total of 1280 blocks or 1.3M. If the map grows to have more blocks the map changes to a doubly indirect map. Here the block addressed from the inode holds block numbers to singly indirect blocks.

Using the RFS

The easiest way to use the RFS and to play with it is to enable the shell in your application. To do this add to the file that handles your {{{condefs.h</code> defines:


In the main of your application start the shell then at the prompt list the devices you have. The RFS test application running on a MCF5235 lists 3 block type devices:

RTEMS SHELL (Ver.1.0-FRC):/dev/console. Feb 20 2010. 'help' to list commands. / # ls -las dev total 0 0 crwxr-xr-x 1 root root 0, 0 Jan 1 00:00 console 0 brwxr-xr-x 1 root root 3, 0 Jan 1 00:00 fdda 0 brwxr-xr-x 1 root root 4, 0 Jan 1 00:00 nvda 0 brwxr-xr-x 1 root root 2, 0 Jan 1 00:00 rda 0 crwxr-xr-x 1 root root 0, 1 Jan 1 00:00 tty01

For this example we will use the flash disk. The flash disk on the MCF5235 lives on the single flash device on the board which is shared with the boot monitor that comes with the board. We assume this is a new board so erase the flash:

/ # fderase /dev/fdda erase flash disk: /dev/fdda flash disk erased successful

The {{{fderase</code> command can be found in the RFS test application. Next we format the disk with the RFS format command. This command is:

<blockquote> <b>mkrfs [-v] [-s blksz] [-b grpblk] [-i grpinode] [-I] [-o %inode] dev</b>
where: ;-v : Verbose. ;-s blksize : The file system block size. ;-b grpblk : Number of blocks in a group. ;-i grpinode : Number of inodes in a group. ;-I : Initialise the inodes. ;-o %inode : The percentage of blocks allocated to inodes in a group. ;dev : The device to format. </blockquote>

Next we format the flash disk with the default configuration parameters then mounting it:

/ # mkrfs /dev/fdda / # mkdir f / # mount -t rfs /dev/fdda /f mounted /dev/fdda -> /f / # ls f /

To see the format of the disk run the {{{debugrfs</code> command with the {{{data</code> sub-command:

/ # debugrfs /f data RFS Filesystem Data

flags: 00000000

blocks: 1510

block size: 1024

size: 1546240

media block size: 512

media size: 1546240

inodes: 1458

bad blocks: 0

max. name length: 512

groups: 1

group blocks: 8192 group inodes: 1458

inodes per block: 18 blocks per block: 256

singly blocks: 1280

doublly blocks: 327680

max. held buffers: 5

blocks used: 84 (5.5%) inodes used: 1 (0.0%)