This document was published in the
June 2000 Daemonnews issue,
it was slightly updated since then.
Ext2fs rev 1 changes in NetBSD
Hubert Feyrer, Manuel Bouyer; May 2000
Manuel Bouyer has made changes to the NetBSD ext2fs to
include changes for revision 1 of the Linux Extended2
filesystem. Here's an introduction on what ext2fs is, an
overview of the first revision's changes, and a short history
of NetBSD's GPL-free ext2fs implementation.
Why so many filesystems?
Running on a broad range of hardware platforms not only means being
able to talk to the hardware. Often it's necessary to access programs
and files installed on machines prior to the transition to another
operating system, or even after.
While changing to (or even playing with) another operating system,
copying all data over to the new filesystem format is often not an
option. And even if so, some method of accessing data is required.
Given that systems may have
their own filesystem format to store data in their "native" format,
accessibility via an appropriate filesystem is required to get to the
data. It is the filesystem's responsibility to understand the layout
of the data on the disk, and to make it accessible for reading and
Besides native filesystems like FFS and LFS, NetBSD comes with a wide
range of file systems which ensure it can read and/or write disks
containing data in the following filesystems:
The filesystem used natively in NetBSD is the Berkeley FFS. The name
was changed from Unix File System (UFS) to avoid the trademarked term
"Unix". See  for more information on the Berkeley FFS.
- Commodore Amiga AmigaDOS
- Acorn Filecore
- MS-DOS FAT, also with Win95 long filenames
- Windows NT NTFS
- ISO 9660 as found on CDROMs and DVDs, with both Rockridge and
Joliet extensions for "long" names.
- Berkeley-based Fast File-System used by Sun's SunOS and
Solaris as well as SGI's Iris
- Linux ext2fs
There were some changes to the ext2fs code used in NetBSD, we'll
talk about that here. After a general overview of ext2fs, we'll
outline the changes made in ext2fs revision 1.
What is ext2fs?
The Second Extended Filesystem (ext2fs) is used as the Linux operating
system's native filesystem format today. Ext2fs is based on the Minix
filesystem, and several improvements were made to make it fit
today's data storage requirements, using semantics and concepts found
then (and today) in the traditional Berkeley based Fast Filesystem (FFS).
Ext2fs uses an on-disk structure similar to the FFS one: the superblock and
inode have roughly the same members (some of them only differ by size, e.g
uid and gid are 32 bits in FFS and 16 in ext2fs), ext2fs directory entries are
identical to the 4.2BSD FFS one, the only difference being that ext2fs doesn't
include the null character at the end of string where FFS does.
4.4BSD FFS includes the file type in directory entries; NetBSD supports both.
The main difference between
ext2fs and FFS is the block allocation policy: ext2fs doesn't have fragments
and tries to always allocate blocks continously where FFS has a upper bound
on the number of contigous blocks allocated for a file.
NetBSD already supports several FFS-derived file systems (FFS, LFS and MFS),
with some functionality (inode manipulation, file lookup, ...) handled by
shared code. The idea was to handle ext2fs as another FFS-derived file system.
Most of the code was derived from FFS; it was just a matter of changing
the structure name, and the name of structure members (and of course the name
of the function). The only part that was written from scratch is the block
Please refer to  for more details on ext2fs.
Changes in ext2fs rev. 1
Ext2fs rev 1 has bitfields in the superblock indicating which features
the ext2fs has. They are categorized in three classes: 'compat' are
features which can be ignored by kernels which don't support it;
'rocompat' are features which can be ignored by kernels which don't
support them if the filesystem is mounted read-only; and 'incompat'
are features which prevent the filesystem from being mounted at all if
the kernel doesn't support one of them.
As of linux-2.2.14 the only 'compat' feature defined so far is
'prealloc', which affects the way blocks are pre-allocated on write;
No support for this optimisation is present in NetBSD so far.
Three 'rocompat' features are defined:
Linux-2.2.14 supports sparse_super and largefile; NetBSD only supports
'sparse_super' (lacking acces to 64bit Linux systems, it's not
known if linux-2.2.14 completely supports largefile, and mke2fs 1.18
doesn't support it either, at least it's not documented in the man
- 'sparse_super': don't copy superblocks in each cylinders groups
(save some space)
- 'largefile': supports files > 2G on 64bit systems (32bit systems
like i386 are still limited to 2G files)
- 'btreedir': suspectedly use btree format for directories (instead
of a flat list)
Two 'incompat' features are defined:
As of linux-2.2.14/e2fsprogs 1.18 only 'filetype' is supported; NetBSD
supports it as well.
- 'compression': A compressing filesystem
- 'filetype': store file types in directory entries; really, really
close to what FFS does since at least 4.4BSD
(you'll have to check that).
History of a non-GPL'd ext2fs implementation
After reading the ext2fs specs, the NetBSD ext2fs started with a 'cp
-r ffs ext2fs' and then (after renaming all functions/defines)
adapting it to ext2fs: no fragments, different block allocation
policy, minor changes in inodes and directory formats (ext2fs is
really close to FFS), byte swapping on big-endian systems, deleting the
'file type in directory entries' code. That code was added back now,
three years later, in Rev. 1 of ext2fs.
Note that the allocation policy doesn't have to be strictly identical
to the Linux one; only the filesystem layout matters, and the specs
were enough for this. By working strictly according to the ext2fs
specs and not looking at/using the GPL'ed code, it was possible to get
a completely GPL-free ext2fs implementation which fits the NetBSD
requirement to keep GPL'ed code out of the kernel.
- M. McKusick, K. Bostic, M. Karels, J. Quarterman: The Design and
Implementation of the 4.4BSD Operating System; Addison-Wesley
Publishing Company, Reading, MA. April 1996, ISBN 0-201-54979-4.
- R. Card, T. Ts'o, S. Tweedie: Design and
Implementation of the Second Extended Filesystem;
- A collection of ext2fs related documents can be found at ftp://sunsite.unc.edu/pub/Linux/system/filesystems/ext2/.
This text was written for DaemonNews.
$Id: ext2fs-rev1.html,v 1.9 2006/02/20 15:19:51 feyrer Exp feyrer $