The original version of this article was first published on IBM
developerWorks, and is property of Westtech Information Services. This
document is an updated version of the original article, and contains
various improvements made by the Gentoo Linux Documentation team.
This document is not actively maintained.
Advanced Filesystem Implementor's Guide : Introducing ext3
In the past few installments, we've taken a bit of a detour by looking at
non-traditional filesystems such as tmpfs and devfs. Now, it's time to get back
to disk-based filesystems, and we do this by taking a look at ext3. The ext3
filesystem, designed by Dr. Stephen Tweedie, is built on the framework of the
existing ext2 filesystem; in fact, ext3 is very similar to ext2 except for one
small (but important) difference -- it supports journaling. Yet even with this
small addition, I think you'll find that that ext3 has several surprising and
intriguing capabilities. In this article, I'll give you a good understanding
of how ext3 compares to the other journaling filesystems currently available.
In my next article, we'll get ext3 up and running.
So, how does ext3 compare to ReiserFS? In previous articles, I explained how
ReiserFS is well suited to handling small files (under 4K), and in certain
situations, ReiserFS' small file performance is ten to fifteen times greater
than that of ext2 and ext3. However, while ReiserFS has many strengths, it also
has weaknesses. In the current implementation of ReiserFS (version 3.6),
certain file access patterns can actually result in significantly worse
performance than ext2 and ext3, particularly when reading large mail
directories. Also, ReiserFS doesn't have a good track record of NFS
compatibility and has poor sparse file performance. In contrast, ext3 is a very
well-rounded filesystem. It's a lot like ext2; it's not going to give you the
blazingly fast small-file performance that ReiserFS gives you, but it's not
going to give you any unexpected performance or functionality hiccups either.
One of the nice things about ext3 is that because it is based on the ext2 code,
ext2 and ext3's on-disk format is identical; this means that a cleanly
unmounted ext3 filesystem can be remounted as an ext2 filesystem with
absolutely no problems. And that's not all. Thanks to the fact that ext2 and
ext3 use identical metadata, it's possible to perform in-place ext2 to ext3
filesystem upgrades. Yes, you read that right. By upgrading a few key system
utilities, installing a modern 2.4 kernel and typing in a single tune2fs
command per filesystem, you can convert your existing ext2 servers into
journaling ext3 systems. You can even do this while your ext2 filesystems are
mounted. The transition is safe, reversible, and incredibly easy, and unlike a
conversion to XFS, JFS, or ReiserFS, you don't need to back up and recreate
your filesystems from scratch. Now, for a moment, consider the thousands of
production ext2 servers in existence that are just minutes away from an ext3
upgrade; then, you'll have a good grasp of ext3's importance to the Linux
If I had to describe ext3 in one word, I'd call it "comfortable". It's
incredibly easy to ext3-enable an existing ext2 system, and after you do,
you're not going to run into any unexpected performance quirks. And there's yet
another way that ext3 excels in the comfort department; ext3 happens to be one
of the most reliable journaled filesystems available for Linux, as I explain
In addition to being ext2-compatible, ext3 inherits other benefits by sharing
ext2's metadata format. For one, ext3 users gain access to a rock-solid fsck
tool. You'll recall that one of the points of using a journaling filesystem is
to avoid the need for an exhaustive fsck in the first place; however if you do
end up getting corrupt metadata, either from a flaky kernel, bad hard drive, or
something else, you'll greatly appreciate the fact that ext3 inherits ext2's
fsck. In contrast, ReiserFS' fsck is in its infancy, and fixing flaky metadata
when it does show up can be a difficult and dangerous process.
Interestingly, ext3 handles journaling very differently than ReiserFS and other
journaling filesystems do. With ReiserFS, XFS, and JFS, the filesystem driver
journals metadata, but makes no provisions for journaling data. With
metadata-only journaling, your filesystem metadata is going to be rock solid,
and you will probably never need to perform an exhaustive fsck. However,
unexpected reboots and system lock-ups can result in significant corruption of
recently-modified data. Ext3 uses a couple of innovative solutions to avoid
these problems, which we'll look at in a bit.
But first, it's important to understand exactly how metadata-only journaling
could end up biting you. As an example, let's say that you were modifying a
file called /tmp/myfile.txt when the machine unexpectedly locked up, forcing a
reboot. If you were using a metadata-only journaling filesystem such as
ReiserFS, XFS or JFS, your filesystem metadata would be easily repaired, thanks
to the metadata journal, and you wouldn't need to sit through a laborious fsck.
However, there's the distinct possibility that when you load /tmp/myfile.txt
into a text editor, your file will not simply be missing recent changes, but
will contain a good amount of garbage and may even be completely unreadable.
This isn't something that will always happen, but it could happen and often
Here's why. Typical journaled filesystems like ReiserFS, XFS, and JFS take
extra special care of metadata, but don't pay too much attention to data. In
our above example, the filesystem driver was in the process of modifying
several filesystem blocks. The filesystem driver updated the appropriate
metadata, but didn't have time to flush the data from its caches to the new
blocks on disk. Thus, when you loaded up /tmp/myfile.txt into a text editor,
part or all of the file contained garbage -- blocks of data that didn't get
initialized in time before the system locked up.
The ext3 approach
Now that we have a good general understanding of this problem, let's look how
ext3 implements journaling. In ext3, the journaling code uses a special API
called the Journaling Block Device layer, or JBD. The JBD has been designed for
the express purpose of implementing a journal on any kind of block device.
Ext3 implements its journaling by "hooking in" to the JBD API. For example, the
ext3 filesystem code will inform the JBD of modifications it is performing, and
will also request permission from the JBD before modifying certain data on
disk. By doing so, the JBD is given the appropriate opportunities to manage the
journal on behalf of the ext3 filesystem driver. It's quite a nice
arrangement, and because the JBD is being developed as a separate, generic
entity, it could be used to add journaling capabilities to other filesystems in
Here are a couple of neat things about the JBD-managed ext3 journal. For one,
ext3's journal is stored in an inode -- a file, basically. Depending on how you
"ext3-enable" your filesystem, you may or may not be able to see this file,
located at /.journal. Of course, by storing the journal in an inode, ext3 is
able to add the needed journal to the filesystem without requiring incompatible
extensions to the ext2 metadata. This is one of the key ways that an ext3
filesystem maintains backwards compatibility with ext2 metadata, and in turn,
the ext2 filesystem driver.
Different journaling approaches
Not surprisingly, it turns out that there are a number of ways to implement a
journal. For example, a filesystem developer could design a journal that stores
spans of bytes that need to be modified on the host filesystem. The advantage
of this approach is that your journal would be able to store lots of tiny
little modifications to the filesystem in a very efficient way, since it would
only record the individual bytes that need to be modified and nothing more.
JBD takes another, and in some ways better, approach. Rather than recording
spans of bytes that must be changed, JBD stores the complete modified
filesystem blocks themselves. The ext3 filesystem driver also uses this
approach and stores complete replicas of the modified blocks (either 1K, 2K, or
4K) in memory to track pending IO operations. At first, this may seem a bit
wasteful. After all, complete blocks contain modified data but may also contain
unmodified (already on disk) data as well.
The approach that the JBD uses is called physical journaling, which means that
the JBD uses complete physical blocks as the underlying currency for
implementing the journal. In contrast, the approach of only storing modified
spans of bytes rather than complete blocks is called logical journaling, and is
the approach used by XFS. Because ext3 uses physical journaling, an ext3
journal will have a larger relative on-disk footprint than, say, an XFS
journal. But because ext3 uses complete blocks internally and in the journal,
ext3 doesn't deal with as much complexity as it would if it were to implement
logical journaling. In addition, the use of full blocks allows ext3 to perform
some additional optimizations, such as "squishing" multiple pending IO
operations within a single block into the same in-memory data structure. This,
in turn, allows ext3 to write these multiple changes to disk in a single write
operation, rather than many. In addition, because the literal block data is
stored in memory, little or no massaging of the in-memory data is required
before writing it to disk, greatly reducing CPU overhead.
Ext3, protector of data
And now, we finally get to see how the ext3 filesystem effectively provides
both metadata and data journaling, avoiding the data corruption problem I
described earlier in this article. In fact, ext3 actually has two methods to
ensure data and metadata integrity.
Originally, ext3 was designed to perform full data and metadata journaling. In
this mode (called "data=journal" mode), the JBD journals all changes to the
filesystem, whether they are made to data or metadata. Because both data and
metadata are journaled, JBD can use the journal to bring both metadata and data
back to a consistent state. The drawback of full data journaling is that it can
be slow, although you can reduce the performance penalty by setting up a
relatively large journal.
Recently, a new journaling mode has been added to ext3 that provides the
benefits of full journaling but without introducing a severe performance
penalty. This new mode works by journaling metadata only. However, the ext3
filesystem driver keeps track of the particular data blocks that correspond
with each metadata update, grouping them into a single entity called a
transaction. When a transaction is applied to the filesystem proper, the data
blocks are written to disk first. Once they are written, the metadata changes
are then written to the journal. By using this technique (called "data=ordered"
mode), ext3 can provide data and metadata consistency, even though only
metadata changes are recorded in the journal. ext3 uses this mode by default.
These days, a lot of people are trying to determine which Linux journaling
filesystem is "best". In truth, there is no one "right" filesystem for every
application; each one has its own strengths. This is one of the benefits from
having so many next-generation Linux filesystems from which to choose. So,
instead of picking an arbitrary "best" filesystem and using it for every
conceivable application, it's far preferable to understand each filesystem's
strengths and weaknesses so that you can make an educated decision as to which
one to use.
Ext3 has a number of strengths. It has been designed to be extremely easy to
deploy. It's based on the solid ext2 filesystem code and it inherits a great
fsck tool. And ext3's journaling capabilities have been specially designed to
ensure the integrity of both metadata and data. All in all, ext3 is a truly
great filesystem, and a worthy successor to the now-venerable ext2 filesystem.
Join me in my next article, when we get ext3 up and running. Until then, you
may want to check out the following resources.
Read a complete
transcript of Dr. Stephen Tweedie's Ext3, Journaling Filesystem
presentation, which was featured at the Ottawa Linux Symposium in July 2000.
Find out more about using ext3 with 2.4 kernels at Andrew Morton's ext3 for 2.4
page. Andrew Morton is the man responsible for porting ext3 to the 2.4 kernel,
and provided invaluable assistance in writing this article. If you can't wait
until my next article, Andrew has a very nice ext3 and 2.4
usage page that will show you how to get ext3 up and running on your
system in no time.
To keep abreast of the latest ext3 developments, be sure to visit the ext3-users mailing list
archive. Of course, you can also subscribe.
Take Daniel Robbins' free JFS
fundamentals tutorial on developerWorks.
Linux resources on developerWorks.
Browse more Open
source resources on developerWorks.
About the author
Residing in Albuquerque, New Mexico, Daniel Robbins is the President/CEO of
Gentoo Technologies, Inc., the creator of Gentoo Linux, an advanced Linux for
the PC, and the Portage system, a next-generation ports system for Linux. He
has also served as a contributing author for the Macmillan books Caldera
OpenLinux Unleashed, SuSE Linux Unleashed, and Samba Unleashed. Daniel has been
involved with computers in some fashion since the second grade, when he was
first exposed to the Logo programming language as well as a potentially
dangerous dose of Pac Man. This probably explains why he has since served as a
Lead Graphic Artist at SONY Electronic Publishing/Psygnosis. Daniel enjoys
spending time with his wife, Mary, and their daughter, Hadassah. You can
contact Daniel at firstname.lastname@example.org.
Page updated October 9, 2005
With the 2.4 release of Linux come a host of new filesystem possibilities,
including Reiserfs, XFS, GFS, and others. These filesystems sound cool, but
what exactly can they do, what are they good at, and exactly how do you go
about safely using them in a production Linux environment? Daniel Robbins
answers these questions by showing you how to set up these new advanced
filesystems under Linux 2.4. In this installment, Daniel takes a look at ext3,
a new improved version of ext2 with journaling capabilities.
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