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Advanced filesystem implementor's guide, Part 8

Content:

1.  Introduction

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.  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.

3.  Laptops...beware?

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.

4.  Journaling options and write latency

Ext3 allows you to choose from one of three data journaling modes at filesystem mount time: data=writeback, data=ordered, and data=journal.

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 -o data=journal command-line 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

In 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

In 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 data=writeback mode and most other journaled filesystems, and it does so without requiring full data journaling. In general, data=ordered 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 well. Instead, 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

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.

Theoretically, 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:

Code Listing 4.1: Rapid writing

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:

Code Listing 4.2: 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):

Written-to-filesystem 16-meg-read-time (seconds)
ext2 78
ReiserFS 67
ext3 data=ordered 93
ext3 data=writeback 74
ext3 data=journal 7

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 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 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=writeback and data=ordered modes see some benefit as well.

5.  data=journal tweaks

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:

Code Listing 5.1: Tweaking bdflush

$ 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.

6.  Conclusion

We've now concluded our coverage of ext3. Join me in my next article as we explore the many wonders of... XFS!

7.  Resources



Print

Page updated October 9, 2005

Summary: With the 2.4 release of Linux come many 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 continues his look at ext3, a new improved version of ext2 with journaling capabilities. He reveals all the inside information on ext3, and demonstrates some shockingly good ext3 data=journal interactive performance numbers.

Daniel Robbins
Author

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