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Reiserfs or ext3: Which journaling filesystem is right for you?

Should you use Reiserfs or the new ext3? For most people it doesn't matter.

(LinuxWorld) -- I just converted VarLinux.org from using the Reiserfs filesystem to the ext3 journaling filesystem. I'm a big fan of Reiserfs, and I haven't changed my opinion of the filesystem. I simply want to kick the tires on ext3 and take it around the block a few times to see what there is to like or dislike.

There are many technical differences between the two filesystems, but there are only three differences that most people should care about. First, you can do an in-place conversion from ext2, which until recently was the default filesystem for most Linux distributions. Second, you can set ext3 to journal data as well as metadata. Today, Reiserfs journals metadata only.

Let's look at these first two differences before we get to the third, because the third deserves special attention.

The in-place conversion of ext2 systems is a nice win for ext3. It obviously wasn't useful in my case since my system was built on Reiserfs, but I can see where a lot of users won't even think about trying any other journaling filesystem besides ext3, simply because it's easier to move from ext2 to ext3 than from ext2 to any other filesystem.

ext3 also sounds like it offers a lot more safety. Here's the difference between data and metadata journaling. Take two computers. Set up one to journal metadata, and the other journal both metadata and data. Start up some automated applications so both computers are making modifications to some data files. Now pull the power plug on both. When you boot up the first computer, it will restore the filesystems to a consistent state quickly. You will almost certainly lose some of the data you were working on when you shut off the power. The second computer should quickly restore your filesystem to a consistent state and preserve all the changes you made to the data before you pulled the plug.

Naturally, the latter computer is the one most people will want, right? Not necessarily. Journaling data in addition to metadata is a costly operation in terms of speed. In most cases, Reiserfs is already faster than ext2 without journaling at all. Reiserfs is usually significantly faster than ext3 with metadata journaling. Reiserfs is dramatically faster than ext3 if you have ext3 doing both metadata and data journaling.

Speed doesn't kill, it bores

Will the cost in terms of performance really matter to most people? Many of us take it for granted that people are consumed with having the fastest computer possible. I've been struggling with this question for years. I wrote a ton of product reviews for InfoWorld starting as far back as 1983, if I'm reckoning my dates correctly. During that time, I designed benchmarks and wrote the test code to exercise many products. My specialty was databases, but I also wrote benchmarks for compilers, spreadsheets, drawing programs, and other product types.

If InfoWorld stood for one thing, it was real-world testing. Our benchmarks were not academic in any sense of the word. We designed them to stress products the way people used them. Nevertheless, as the years passed, most of the benchmarks we created and ran seemed increasingly pointless. Did anyone care anymore if a program could recalculate a large spreadsheet in 2.5 seconds or 4 seconds? I certainly didn't. Why would InfoWorld readers care? The majority of these folks adopted Microsoft Office simply because it was the office suite everyone else seemed to be using.

This phenomenon is not unique to the Windows world. Many Linux users twiddle their thumbs waiting for StarOffice to load. It takes much longer to load than KWord or AbiWord, but they continue to use StarOffice anyway because they're used to it. Sure, it would be nice if StarOffice loaded more quickly. Most people think there's nothing wrong with slow performance as long as the speed is below the threshold of intolerability.

More important, most of us tend to throw hardware at a speed problem, not software. Hard-core gamers buy the latest Athlon chips and video cards and overclock them. When the VarLinux.org Web site was "slashdotted" some months back, I didn't tune the filesystem or adjust the way Apache worked. I could have. It was much easier to throw $50 at the problem by purchasing another 256-megabytes DIMM.

Before you flame me for what I'm about to say, let me be clear I'm not suggesting Linus Torvalds should start accepting sloppy and bloated code for the kernel simply because you can throw a faster processor and more memory at Linux to compensate. There is virtue in doing the right thing just for the sake of doing it right. That attitude usually pays off in the end.

If someone wants the safety of data journaling that ext3 offers and is unhappy with the tradeoff in speed, then I don't see anything wrong with purchasing a faster drive to compensate for the performance loss.

This holds true for Reiserfs as well. If you insist on using Reiserfs but are paranoid about the possibility of losing data, you can always turn off the write-caching feature of your disk drive with a command like /sbin/hdparm -W0 /dev/hda or something similar, depending on how your system is set up. (The sample command is for the first IDE drive in your system.) Your system performance will take a nosedive if you do something like that, but then you were willing to tolerate the performance hit from ext3 data journaling, so why not? Indeed, I have write caching turned off on one drive on one of my systems because I'm paranoid about losing data on that drive. The performance hit is terrible, but I believe it's worth the pain.

The point, if there is one, is that I don't think performance is going to be the determining factor when most people choose between Reiserfs, ext3, or any other filesystem. It may be the deal-breaker for speed freaks and special circumstances where speed is critical. In addition, ext3 has the distinct advantage of letting you control data journaling per mount point. In other words, you can mount multiple partitions from a single drive and choose to journal data for one partition but not another. That's not an option if you turn off write caching for the whole drive.

Regardless, I believe that we humans are a lazy lot, and therefore most folks won't even think much about performance or data safety and will go the path of least resistance when choosing a filesystem. Where that path leads depends on what you're using now, the next Linux distribution you choose, whether you install it fresh or upgrade a system, and how easy the distribution makes the process of choosing a filesystem format.

Curtain number three

This brings us to factor three -- extensibility.

If you visit the Reiserfs Web site, you'll see that the Reiser folks make a bit of a fuss about space efficiency as well as speed. I noticed I lost free disk space when I converted from Reiserfs to ext3, but disk space, like memory, is cheap these days.

The Reiserfs folks then go on at length about balanced trees and other stuff should interest database or filesystem experts, or anyone else with little or no social life. Having been thrown into the field of database management against my will, I find the information moderately interesting, but not enough to study at length. If you are a member of any of the aforementioned groups, then visit the site and have a look for yourself.

What stands out the most is that the folks who designed Reiserfs did so with an eye toward future plug-ins that will provide features like access control lists, hyperlinks, and other nifty doo-dads.

There are two sides to this promise. Assuming the Reiserfs folks knew what they were doing (and I have no reason to doubt this), Reiserfs is likely a more robust foundation for these added features than ext3. As far as I know, no one planned for journaling when designing ext2. Journaling was bolted on as an afterthought.

That's not to say it was done badly. I'm impressed anyone added both data and metadata journaling to ext2. The bolts for both look sturdy so far. Intuition tells me you can bolt only so much onto an old system before the struts get wobbly and the foundation begins to crack. That's just intuition speaking, but I find that it usually pays to listen to intuition. My intuition says it may be a smart idea to invest in Reiserfs rather than ext3 if you anticipate needing some of the planned plug-ins for Reiserfs.

On the down side, the Web site has tantalized Reiserfs fans with these add-ins for as long as I can remember, and they're still nowhere to be seen. As cool as Reiserfs sounds with respect to extensibility, it would be nice to see these features get past the stage of HTML. As long as it remains a promise, there's no telling if the promise is any better than the HTML it's written on. Planned extensibility is a concept that's easy to promote when you don't have anything to prove its worth. Until we see some real plug-ins, we'll never know if Reiserfs is the ideal foundation upon which to build things like access control lists, or if Reiserfs will crumble under the weight.

In the meantime, I'll continue messing around with ext3, and offer additional observations in future columns.

More Stories By Nicholas Petreley

Nicholas Petreley is a computer consultant and author in Asheville, NC.

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