Common threads

Advanced filesystem implementor's guide, Part 7

Introducing ext3

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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.

Understanding Ext3

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 community.

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 below.

Ext3 reliability

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.

Metadata-only journaling

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 does.

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 the future.

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.

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Zone=Linux, Open source
ArticleTitle=Common threads: Advanced filesystem implementor's guide, Part 7