YAFFS
Encyclopedia
YAFFS was designed and written by Charles Manning, of Whitecliffs
, New Zealand, for the company Aleph One.
Yaffs1 is the first version of this file system and works on NAND chips that have 512 byte
pages + 16 byte spare (OOB;Out-Of-Band) areas. These older chips
also generally allow 2 or 3 write cycles per page, which YAFFS takes advantage of - i.e. dirty pages are marked by writing to a specific spare area byte.
Newer NAND flash chips have larger pages, 2048 bytes + 64 bytes spare areas, and stricter write requirements. Each page within an erase block (128 kilobytes) must be written to in sequential order, and each page must be written only once. YAFFS2 was designed to accommodate these newer chips. YAFFS2 is based on the YAFFS1 source code, with the major difference being that internal structures are not fixed to assume 512 byte sizing, and a block sequence number is placed on each written page. In this way older pages can be logically overwritten without violating the "write once" rule.
YAFFS is a robust log-structured file system
that holds data integrity
as a high priority. A secondary YAFFS goal is high performance. YAFFS will typically outperform most alternatives. It is also designed to be portable and has been used on Linux
, WinCE, pSOS
, eCos
, ThreadX
and various special-purpose OSes. A variant 'YAFFS/Direct' is used in situations where there is no OS, embedded OSes and bootloaders: it has the same core filesystem but simpler interfacing to the OS and NAND flash hardware.
The filesystem is licensed both under the GPL
and under per-product licenses available from Aleph One.
a YAFFS filesystem beyond simply erasing the flash memory. When a bad block is encountered, YAFFS follows the smart media scheme of marking the fifth byte of the block's spare area. Blocks marked as such remain unallocated from then on.
To write file data, YAFFS initially writes a whole page (chunk in YAFFS terminology) that describes the file metadata
, such as timestamp
s, name, path, etc. The new file is assigned a unique object ID number; every data chunk within the file will contain this unique object ID within the spare area. YAFFS maintains a tree structure in RAM
memory of the physical location of these chunks. When a chunk is no longer valid (the file is deleted, or parts of the file are overwritten), YAFFS marks a particular byte in the spare area of the chunk as ‘dirty’. When an entire block (32 pages) is marked as dirty, YAFFS can erase the block and reclaim the space. When the filesystem's free space is low, YAFFS consolidates a group of good pages onto a new block. YAFFS then reclaims the space used by dirty pages within each of the original blocks.
When a YAFFS system mount
s a NAND flash device, it must visit each block to check for valid data by scanning its spare area. With this information it then reconstitutes the memory-resident tree data structure.
YAFFS2 marks every newly written block with a sequence number that is monotonically increasing
. The sequence of the chunks can be inferred from the block sequence number and the chunk offset within the block. Thereby when YAFFS2 scans the flash and detects multiple chunks that have identical ObjectIDs and ChunkNumbers, it can choose which to use by taking the greatest sequence number. For efficiency reasons YAFFS2 also introduces the concept of shrink headers. For example when a file is resized to a smaller size, YAFFS1 will mark all of the affected chunks as dirty - YAFFS2 cannot do this due to the "write once" rule. YAFFS2 instead writes a "shrink header", which indicates that a certain number of pages before that point are invalid. This lets YAFFS2 reconstruct the final state of the filesystem when the system reboots.
YAFFS2 uses a more abstract definition of the NAND flash allowing it to be used with a wider variety of flash parts with different geometries, bad block handling rules etc.
YAFFS2 now supports "checkpointing" which bypasses normal mount scanning, allowing very fast mount times. Mileage will vary, but mount times of 3 seconds for 2 GB
have been reported.
Whitecliffs
Whitecliffs is a small town with a population of 117 in the Selwyn District of the Canterbury region of New Zealand's South Island. It has also been known as South Malvern , and the name of Whitecliffs comes from terrace cliffs above the Selwyn River...
, New Zealand, for the company Aleph One.
Yaffs1 is the first version of this file system and works on NAND chips that have 512 byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
pages + 16 byte spare (OOB;Out-Of-Band) areas. These older chips
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
also generally allow 2 or 3 write cycles per page, which YAFFS takes advantage of - i.e. dirty pages are marked by writing to a specific spare area byte.
Newer NAND flash chips have larger pages, 2048 bytes + 64 bytes spare areas, and stricter write requirements. Each page within an erase block (128 kilobytes) must be written to in sequential order, and each page must be written only once. YAFFS2 was designed to accommodate these newer chips. YAFFS2 is based on the YAFFS1 source code, with the major difference being that internal structures are not fixed to assume 512 byte sizing, and a block sequence number is placed on each written page. In this way older pages can be logically overwritten without violating the "write once" rule.
YAFFS is a robust log-structured file system
Log-structured file system
A log-structured filesystem is a file system design first proposed in 1988 by John K. Ousterhout and Fred Douglis. Designed for high write throughput, all updates to data and metadata are written sequentially to a continuous stream, called a log...
that holds data integrity
Data integrity
Data Integrity in its broadest meaning refers to the trustworthiness of system resources over their entire life cycle. In more analytic terms, it is "the representational faithfulness of information to the true state of the object that the information represents, where representational faithfulness...
as a high priority. A secondary YAFFS goal is high performance. YAFFS will typically outperform most alternatives. It is also designed to be portable and has been used on Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, WinCE, pSOS
PSOS
pSOS is a real time operating system , created in about 1982 by Alfred Chao, and developed/marketed for the first part of its life by his company Software Components Group...
, eCos
ECos
eCos is an open source, royalty-free, real-time operating system intended for embedded systems and applications which need only one process with multiple threads. It is designed to be customizable to precise application requirements of run-time performance and hardware needs...
, ThreadX
ThreadX
ThreadX, developed and marketed by Express Logic, Inc. of San Diego, California, USA, is a real-time operating system . Similar RTOSes are available from other vendors such as VxWorks, Nucleus RTOS, OSE, QNX, LynxOS, etc...
and various special-purpose OSes. A variant 'YAFFS/Direct' is used in situations where there is no OS, embedded OSes and bootloaders: it has the same core filesystem but simpler interfacing to the OS and NAND flash hardware.
The filesystem is licensed both under the GPL
GNU General Public License
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
and under per-product licenses available from Aleph One.
YAFFS1
There is no special procedure to initializeDisk formatting
Disk formatting is the process of preparing a hard disk drive or flexible disk medium for data storage. In some cases, the formatting operation may also create one or more new file systems...
a YAFFS filesystem beyond simply erasing the flash memory. When a bad block is encountered, YAFFS follows the smart media scheme of marking the fifth byte of the block's spare area. Blocks marked as such remain unallocated from then on.
To write file data, YAFFS initially writes a whole page (chunk in YAFFS terminology) that describes the file metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
, such as timestamp
Timestamp
A timestamp is a sequence of characters, denoting the date or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself...
s, name, path, etc. The new file is assigned a unique object ID number; every data chunk within the file will contain this unique object ID within the spare area. YAFFS maintains a tree structure in RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
memory of the physical location of these chunks. When a chunk is no longer valid (the file is deleted, or parts of the file are overwritten), YAFFS marks a particular byte in the spare area of the chunk as ‘dirty’. When an entire block (32 pages) is marked as dirty, YAFFS can erase the block and reclaim the space. When the filesystem's free space is low, YAFFS consolidates a group of good pages onto a new block. YAFFS then reclaims the space used by dirty pages within each of the original blocks.
When a YAFFS system mount
Mount (computing)
Mounting takes place before a computer can use any kind of storage device . The user or their operating system must make it accessible through the computer's file system. A user can access only files on mounted media.- Mount point :A mount point is a physical location in the partition used as a...
s a NAND flash device, it must visit each block to check for valid data by scanning its spare area. With this information it then reconstitutes the memory-resident tree data structure.
YAFFS2
YAFFS2 is similar in concept to YAFFS1, and shares much of the same code; the YAFFS2 code base supports YAFFS1 data formats through backward compatibility. The main difference is that YAFFS2 needs to jump through significant hoops to meet the "write once" requirement of modern NAND flash.YAFFS2 marks every newly written block with a sequence number that is monotonically increasing
Monotonic function
In mathematics, a monotonic function is a function that preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory....
. The sequence of the chunks can be inferred from the block sequence number and the chunk offset within the block. Thereby when YAFFS2 scans the flash and detects multiple chunks that have identical ObjectIDs and ChunkNumbers, it can choose which to use by taking the greatest sequence number. For efficiency reasons YAFFS2 also introduces the concept of shrink headers. For example when a file is resized to a smaller size, YAFFS1 will mark all of the affected chunks as dirty - YAFFS2 cannot do this due to the "write once" rule. YAFFS2 instead writes a "shrink header", which indicates that a certain number of pages before that point are invalid. This lets YAFFS2 reconstruct the final state of the filesystem when the system reboots.
YAFFS2 uses a more abstract definition of the NAND flash allowing it to be used with a wider variety of flash parts with different geometries, bad block handling rules etc.
YAFFS2 now supports "checkpointing" which bypasses normal mount scanning, allowing very fast mount times. Mileage will vary, but mount times of 3 seconds for 2 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
have been reported.
See also
- List of file systems
- JFFSJFFSThe Journaling Flash File System is a log-structured file system for use on NOR flash memory devices on the Linux operating system. It has been superseded by JFFS2.- Design :...
- JFFS2JFFS2Journalling Flash File System version 2 or JFFS2 is a log-structured file system for use with flash memory devices. It is the successor to JFFS. JFFS2 has been included in the Linux kernel since the 2.4.10 release. JFFS2 is also available for a couple of bootloaders like Das U-Boot, Open...
- UBIFSUBIFSThe Unsorted Block Image File System is a successor to JFFS2, and competitor to LogFS, as a file system for use with raw flash memory media. Development began in earnest in 2007, with the first stable release made to Linux kernel 2.6.27 in October 2008.Note that UBIFS works on top of an Unsorted...
- LogFSLogFSLogFS is a Linux log-structured and scalable flash file system, intended for use on large devices of flash memory. It is written by Jörn Engel and in part sponsored by the CE Linux Forum....
- Open NAND Flash Interface Working GroupOpen NAND Flash Interface Working GroupThe Open NAND Flash Interface Working Group , is a consortium of technology companies working to develop open standards for NAND flash memory chips and devices that communicate with them...
External links
- YAFFS - Yet Another Flash Filing System main website – a filing system optimised for NAND Flash chips (for embedded use)
- Introducing YAFFS, the first NAND-specific flash file system
- Unyaffs: A simple program to unpack YAFFS2 images.
- yaffs2utils: Utilities to create/extract a YAFFS2 image on Linux.