File Systems

This page has information about file systems which are of interest for embedded projects.

Introduction
Most embedded devices use flash memory as storage media. Also, size and bootup time are very important in many consumer electronics products. Therefore, special file systems are often used with differrent features, such as enhanced compression, or the ability to execute files directly from flash.

MTD
Note that flash memory may be managed by the Memory Technology Devices (MTD) system of Linux. See the MTD/Flash FAQ for more information. Most of the filesystems mentioned here are built on top of the MTD system.

UBI
The Unsorted Block Images (UBI) system in the Linux kernel manages multiple logical volumes on a single flash device. It provides a mapping from logical blocks to physical erase blocks, via the MTD layer. UBI provides a flexible partitioning concept which allows for wear-leveling across the whole flash device.

See the UBI page or UBI FAX and Howto for more information.

Partitioning
The kernel requires at least one "root" file system, onto which other file systems can be mounted. In non-embedded systems, often only a single file system is used. However, in order to optimize limited resources (flash, RAM, processor speed, boot up time), many embedded systems break the file system into separate parts, and put each part on it's own partition (often in different kinds of storage.

For example, a developer may wish to take all the read-only files of the system, and put them into a compressed, read-only file system in flash. This will consume the least amount of space on flash, at the cost of some read-time performance (for decompression).

Another configuration might have executable files stored uncompressed on flash, so that they can be executed-in-place, which saves RAM and boot-up time (with a potential small loss of performance).

For writable data, if the data does not need to be persistent, sometimes a ramdisk is used. Depending on the performance needs and the RAM limits, the file data may be compressed or not.

There is no single standard for interleaving the read-only and read-write portions of the file system. This depends heavily on the set of embedded applications used for the project.

Embedded Filesystems
Here are some filesystems designed for and/or commonly used in embedded devices:

JFFS2

 * JFFS2 - The Journalling Flash File System, version 2. This is the most commonly used flash filesystem.
 * The maximum size of JFFS2 is 128MB.
 * http://sourceforge.net/projects/mtd-mods has some patches by Alexey Korolev for improvements to JFFS2
 * See the presentation on Alexey's patches at:

CramFS

 * CRAMFS - A compressed read-only file system for Linux. The maximum size of CRAMFS is 256MB.
 * "Linear Cramfs" is the name of a special feature to use uncompressed file, in a linear block layout with the Cramfs file system. This is useful for storing files which can be executed in-place.  For more information on Linear Cramfs, see Application XIP

SquashFS

 * Squash Fs - A (more) compressed read-only file system for Linux. This file system has better compression than JFFS2 or CRAMFS.

YAFFS2

 * YAFFS - Yet Another Flash File System - a file system designed specifically for NAND flash
 * Presentation on YAFFS2 by Wookey at ELC Europe 2007: yaffs.pdf
 * Presentation from CELF Jamboree 17 comparing YAFFS and JFFS2 on 2.6.10: celf_flash.pdf

YAFFS vs. JFFS2 mount time comparisons for 2.6.10
Here are some core results for mount times. (See the Toshiba Jamboree17 presentation for details.)


 * hardware: MIPS, 333 MHZ CPU, with 64 MB NAND Flash.
 * kernel: 2.6.10 +EBS patch +YAFFS (20061128 version).
 * JFFS2 compression option is disabled.
 * Key:
 * “Initial”: Time for mounting when the mount is just after launching “flash_eraseall”.
 * "1000 files”: Time for mounting after creating 1000 files (one file size is 33554 bytes.)
 * “JFFS2+EBS” needs to check EBS, and then it does start to scan the blocks normally. Therefore, “Initial” mount time is a little bit slow.

LogFS
JFFS2 for most uses, but focuses more on the large devices.
 * logfs - LogFS is a scalable flash filesystem. It is aimed to replace

Matt Mackall writes (in July of 2007):

LogFS is a filesystem designed to support large volumes on FLASH. It uses a simple copy-on-write update process to ensure consistency (the "log" in the name is a historical artifact). It's easily the most modern and scalable open-source FLASH filesystem available for Linux and it's well on its way to being accepted in the mainline tree.

Scott Preece writes:

The big win for LogFS (in my limited knowledge of it) is that it stores its tree structure in the media, rather than building it in memory at mount time. This significantly reduces both startup time and memory consumption. This becomes more important as the size of the flash device increases. Read more in LWN (http://lwn.net/Articles/234441) and linux.com (http://www.linux.com/articles/114295).

Some newer flash memory, like MLC (multi-level cell), are not well supported.

LogFS now has it's own mailing list: see http://logfs.org/cgi-bin/mailman/listinfo/logfs

AXFS

 * AXFS - Advanced XIP File System
 * This file system is designed specifically to support Execute-in-place operations

NFS
Due to space constraints on embedded devices, it is common during development to use a network file system for the root filesystem for the target. This allows the target to have a very large area where full-size binaries and lots of development tools can be placed during development. One drawback to this approach is that the system will need to be re-configured with local file systems (and most likely re-tested) for final product shipment, at some time during the development cycle.

An NFS client can be built into the Linux kernel, and the kernel can be configured to use NFS as the root filesystem. This requires support for networking, and mechanisms for specifying the IP address for the target, and the path to the filesystem on the NFS host. Also, the host must be configured to run an NFS server. Often, the host also provides the required address and path information to the target board by running a DHCP server.

See the the file Documentation/nfsroot.txt in the Linux kernel source for more information about mounting an NFS root filesystem with the kernel.

Mounting the root filesystem
The root filesystem is mounted by the kernel, using a kernel command line option. Other file systems are mounted from user space, usually by init scripts or an init program, using the 'mount' command.

The following are examples of command lines used for mounting a root filesystem with Linux:


 * Use the first partition on the first IDE hard drive:
 * root=/dev/hda1
 * or in later kernels:
 * root=/dev/sda1


 * Use NFS root filesystem (kernel config must support this)
 * root=/dev/nfs

(Usually you need to add some other arguments to make sure the kernel IP address gets configured, or to specify the host NFS path.)


 * Use flash device partition 2:
 * root=/dev/mtd/2

[FIXTHIS - should probably mention initrd's here somewhere]

ABISS
The Active Block I/O Scheduling System is a file system designed to be able to provide real-time features for file system I/O activities.

See ABISS

UnionFS
Sometimes it is handy to be able to overlay file systems on top of each other. For example, it can be useful in embedded products to use a compressed read-only file system, mounted "underneath" a read/write file system. This give the appearance of a full read-write file system, while still retaining the space savings of the compressed file system, for those files that won't change during the life of the product.

UnionFS is a project to provide such a system (providing a "union" of multiple file systems).

See http://www.filesystems.org/project-unionfs.html

See also union mounts, which are described at http://lkml.org/lkml/2007/6/20/18 (and also in Documentation/union-mounts.txt in the kernel source tree - or will be, when this feature is merged.)

Multi-media file systems

 * XPRESS file system - [See OLS 2006 proceedings, presentation by Joo-Young Hwang]
 * I found out at ELC 2007 that this FS project was recently suspended internally at Samsung