Advanced filesystem implementor's guide, Part 8
Surprises in ext3
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 Related topics 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 Related topics). 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 Related topics).
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
option when calling
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
rootflags. So, if you'd like to put
your root filesystem into full data journaling mode, add
rootflags=data=journal to your kernel boot options.
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 drawback,
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
data=writeback mode and most other journaled
filesystems, and it does so without requiring full data journaling. In
data=ordered ext3 filesystems perform slightly
data=writeback filesystems, but significantly
faster than their full data journaling counterparts.
When appending data to files,
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
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.
data=journal mode 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 overhead):
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.
data=journal mode, 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. Maybe
data=journal mode isn't so sluggish after all!
Andrew is still trying to figure out exactly why
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 well.
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)
- 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.
- 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.