I'm going to be honest. For this article, I was planning to show you how to get ext3 up and running on your system. Although that's what I said I'd do, I'm not going to do it. Andrew Morton's excellent "Using the ext3 filesystem in 2.4 kernels" page (see Resources later in this article) already does a great job of explaining how to ext3-enable your system, so there's no need for me to repeat all the basics here. Instead, I'm going to delve into some meatier ext3 topics, ones that I think you'll find very useful. After you read this article, when you're ready to get ext3 up and running, head over to Andrew's page.
2.4 kernel update
First, let's start with a 2.4 kernel update. I last discussed 2.4 kernel stability when I was covering ReiserFS. Way back then, finding a stable 2.4 kernel was a challenge, and I recommended sticking with the known and at that time bleeding-edge 2.4.4-ac9 kernel -- especially for anyone planning to use the ReiserFS filesystem in a production environment. As you might guess, a lot has happened since 2.4.4-ac9, and it's definitely time to start looking at newer kernels.
With kernel 2.4.10, the 2.4 series reached a new level of performance and scalability (something that we've been anticipating for a long time). So, what happened to allow Linux 2.4 to finally grow up? In an acronym, VM. Linus, recognizing that the 2.4 series wasn't performing spectacularly, ripped out Linux's problematic VM code and replaced it with a lean and mean VM implementation from Andrea Archangeli. Andrea's new VM implementation (which first appeared in 2.4.10) was really great; it really sped up the kernel and made the entire system more responsive. 2.4.10 was definitely a major turning point in 2.4 Linux kernel development; up until then, things weren't looking very good, and many of us were wondering why we weren't FreeBSD developers. We all should thank Linus for his heroism in making such a major (but sorely needed) change in the 2.4 stable kernel series.
Since Andrea's new VM code needed a bit of time to be integrated seamlessly with the rest of the kernel, use 2.4.13+. Even better, use 2.4.16+, since the rock-solid ext3 filesystem code was finally integrated into the official Linus kernel starting with the 2.4.15-pre2 release. There's no reason to avoid using 2.4.16+ kernel, and it'll make your job of getting ext3 up and running that much easier. If you do use a 2.4.16+ kernel, just remember that it's no longer necessary to apply the ext3 patch as described on Andrew's page (see Resources). Linus already added it for you :)
You'll notice that I recommend using 2.4.16+ rather than 2.4.15+, and with good reason. With the release of kernel 2.4.15-pre9, a really ugly filesystem corruption bug was introduced to the kernel. It took until 2.4.16-pre1 for the problem to be identified and fixed, resulting in a span of kernels (including 2.4.15) that should be avoided at all costs. Choosing a 2.4.16+ kernel allows you to avoid this bad batch entirely.
Ext3 has a stellar reputation for being a rock-solid filesystem, so I was surprised to learn that quite a few laptop users were having filesystem corruption problems when they switched to ext3. In general, it's tempting to react to these kinds of reports by avoiding ext3 entirely; however, after asking around, I discovered that the disk corruption problems that people were experiencing had nothing to do with ext3 itself, but were being caused by certain laptop hard drives.
The write cache
You may not know this, but most modern hard drives have something called a "write cache", used by the hard drive to collect pending write operations. By putting pending writes into a cache, the hard drive firmware can then reorder and group them so that they're written to disk in the fastest possible way. The write cache is generally considered to be a very good thing (read Linus' explanation and opinion of write caching in Resources).
Unfortunately, certain laptop hard drives now on the market have the dubious feature of ignoring any official ATA request to flush their write cache to disk. This isn't a wonderful design feature, although it has been allowed by the ATA spec up until recently. With these types of drives, there's no way for the kernel to guarantee that a particular block has actually been recorded to the disk platters. Although this sounds like a thorny problem, this particular issue by itself is probably not the cause of the data corruption problems that people have been experiencing.
However, it gets worse. Some modern laptop hard drives have an even nastier habit of throwing away their write cache whenever the system is rebooted or suspended. Obviously, if a hard drive has both of these problems, it's going to regularly corrupt data, and there's nothing that Linux can do to prevent it from doing so.
So, what's the solution? If you have a laptop, tread carefully. Back up all your important files before making any major change to your filesystems. If you experience data corruption problems that seem to fit the pattern of what I described above, particularly with ext3, then remember that it may be your laptop hard drive that's at fault. In that case, you may want to contact your laptop manufacturer and inquire about getting a replacement drive. Hopefully, in a few months time, these flaky hard drives will be pulled from the market and we'll never need to worry about this issue again.
Now that I've scared you out of your minds, let's take a look at ext3's various data journaling options.
Journaling options and write latency
Ext3 allows you to choose from one of three data journaling modes at
filesystem mount time:
To specify a journal mode, you can add the appropriate string
data=journal, for example) to the options
section of your /etc/fstab, or specify the
data=journal command-line option when
mount directly. If you'd like to
specify the data journaling method used for your root filesystem
data=ordered is the default), you can to use a
special kernel boot option called
So, if you'd like to put your root filesystem into full data journaling
rootflags=data=journal to your kernel
data=writeback mode, ext3 doesn't do any
form of data journaling at all, providing you with similar journaling
found in the XFS, JFS, and ReiserFS filesystems (metadata only). As I
explained in my previous article, this could allow recently modified files to
become corrupted in the event of an unexpected reboot. Despite this
data=writeback mode should give you
the best ext3 performance under most conditions.
data=ordered mode, ext3 only officially
journals metadata, but it logically groups metadata and data blocks into a
single unit called a transaction. When it's time to write the new metadata
out to disk, the associated data blocks are written first.
data=ordered mode effectively solves the
corruption problem found in
and most other journaled filesystems, and it does so without requiring
full data journaling. In general,
ext3 filesystems perform slightly slower than
data=writeback filesystems, but significantly
faster than their full data journaling counterparts.
When appending data to files,
data=ordered mode provides all of the integrity
guarantees offered by ext3's full data journaling mode. However, if part
of a file is being overwritten and the system crashes, it's
possible that the region being written will contain a combination of
original blocks interspersed with updated blocks. This is because
data=ordered provides no guarantees as to which
blocks are overwritten first, so you can't assume that just because
overwritten block x was updated, that overwritten block x-1 was updated as
data=ordered leaves the write
ordering up to the hard drive's write cache. In general, this limitation
doesn't end up negatively impacting people very often, since file appends
are generally much more common than file overwrites. For this reason,
data=ordered mode is a good higher-performance
replacement for full data journaling.
data=journal mode provides full data and
metadata journaling. All new data is written to the journal first, and
then to its final location. In the event of a crash, the journal can be
replayed, bringing both data and metadata into a consistent state.
data=journal mode is the slowest
journaling mode of all, since data gets written to disk twice rather than
once. However, it turns out that in certain situations,
data=journal mode can be blazingly fast. Andrew
Morton, after hearing reports on LKML that ext3
data=journal filesystems were giving people
unbelievably great interactive filesystem performance, decided to put
together a little test. First, he created simple shell script designed to
write data to a test filesystem as quickly as possible:
while true do dd if=/dev/zero of=largefile bs=16384 count=131072 done
While data was being written to the test filesystem, he attempted to read 16Mb of data from another ext2 filesystem on the same disk, timing the results:
Reading a 16Mb file
time cat 16-meg-file > /dev/null
The results were astounding.
allowed the 16-meg-file to be read from 9 to over 13 times faster
than other ext3 modes, ReiserFS, and even ext2 (which has no journaling
Andrew repeated this test, but tried to read a 16Mb file from the test
filesystem (rather than a different filesystem), and he got identical
results. So, what does this mean? Somehow, ext3's
data=journal mode is incredibly well-suited to
situations where data needs to be read from and written to disk at the
same time. Therefore, ext3's
which was assumed to be the slowest of all ext3 modes in nearly all
conditions, actually turns out to have a major performance advantage in
busy environments where interactive IO performance needs to be maximized.
data=journal mode isn't so sluggish after
Andrew is still trying to figure out exactly why
data=journal mode is doing so much better than
everything else. When he does, he may be able to add the necessary tweaks
to the rest of ext3 so that
data=ordered modes see some benefit as
Some people have had a particular performance problem when using ext3's data=journal mode on busy servers -- busy NFS servers, in particular. Every thirty seconds, the server experiences a huge storm of disk-writing activity, causing the system to nearly grind to a halt. If you experience this problem, it's easy to fix. Simply type the following command as root to tweak Linux's dirty buffer-flushing algorithm:
echo 40 0 0 0 60 300 60 0 0 > /proc/sys/vm/bdflush
These new bdflush settings will cause kupdate to run every 0.6 seconds rather than every 5 seconds. In addition, they tell the kernel to flush a dirty buffer after 3 seconds rather than 30, the default. By flushing recently-modified data to disk more regularly, these write storms can be avoided. It's slightly less efficient to do things this way, since the kernel will have fewer opportunities to combine writes. But for a busy server, writes will happen more consistently, and interactive performance will be greatly improved.
We've now concluded our coverage of ext3. Join me in my next article as we explore the many wonders of...XFS!
- Read Daniel's previous articles in his filesystems series on
developerWorks, where he described:
- the benefits of journalling and ReiserFS (Part 1)
- setting up a ReiserFS system (Part 2)
- using the tmpfs virtual memory filesystem and bind mounts (Part 3)
- the benefits of devfs, the device management filesystem (Part 4)
- beginning the conversion to devfs (Part 5)
- completing the conversion to devfs using an init wrapper (Part 6)
- the benefits of the ext3 filesystem (Part 7)
- Visit Andrew Morton's ext3 and 2.4 usage page to complete your ext3 setup.
- Find out more about using ext3 with 2.4 kernels at Andrew Morton's ext3 for 2.4 page.
- Learn more about the strange laptop hard drive corruption problems by reading Kernel Traffic's summary.
- Read Linus' explanation and opinion of write caching.
- Read a complete transcript of Dr. Stephen Tweedie's Ext3, Journaling Filesystem presentation, which was featured at the Ottawa Linux Symposium in July 2000.
- To keep abreast of the latest ext3 developments, be sure to visit the ext3-users mailing list archive. Of course, you can also subscribe.
- Browse more Linux resources on developerWorks.
- Browse more Open source resources on developerWorks.