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Btrfs by default in Fedora 16?

By Jonathan Corbet
February 23, 2011
Btrfs is, by many accounts, the next-generation filesystem for Linux systems. It has, in fact, been the next-generation filesystem for a few years now; progress in the filesystem area often seems to be slow. But btrfs has been maturing to the point that it is highly usable in a number of environments. The list of btrfs users may grow significantly toward the end of this year if a proposal to make it the default filesystem for the Fedora 16 release is adopted.

That proposal was made by btrfs developer Josef Bacik. Josef would not only like to see btrfs as the default F16 filesystem; he would like to stop using the LVM volume manager in favor of the internal volume management built into btrfs. As he notes: "Fedora 16 is a very aggressive target, which is also why I'm bringing it up now. I think we will be ready by then." There are a few things that will have to happen, though, for that to be true.

For example, there is the little problem that there is still no filesystem checker for btrfs. That is, according to btrfs creator Chris Mason, the biggest missing feature in the filesystem at the moment. He also said that he is working on it full time, so this particular gap should be filled sometime in the near future. That said, filesystem checkers, like the filesystems themselves, require a certain amount of time to mature into the kind of rock-solid behavior that users tend to expect. The btrfsck program will certainly have to evolve over time as the various ways in which filesystems can become corrupted are discovered.

Josef noted that support in the GRUB bootloader is another open problem. Patches adding support to GRUB1 exist, but they would have to be carried by Fedora, since there is no functioning upstream for GRUB1. The alternative is to move the distribution to GRUB2, a tool which has the added advantage of an existing upstream development community. Fedora is already contemplating moving to GRUB2, but, with regard to btrfs, there is, naturally, a catch. As was discussed in this article last December, GRUB2 is licensed under GPLv3, while the btrfs code is GPLv2-only. As has been done with ZFS, getting btrfs support into GRUB2 would require relicensing some of the code to GPLv2+. There are now enough contributors to btrfs to make that relicensing an interesting problem, but it is probably still feasible at this point.

Another potential issue is that the developers of Anaconda (the Fedora installer) have complained that they already have a lot of things to work on for the Fedora 16 release. They don't relish the idea of more work, but, according to Chris Lumens, "perhaps we can find some time somewhere." Simply installing to btrfs should be a relatively easy change; reworking volume management to be done within btrfs sounds like rather more work.

Jon Masters opposed the idea of switching away from LVM, saying:

Yes, BTRFS can do a lot of volume-y things, and these are growing by the day, but I don't want my filesystem replacing a full volume manager and I am concerned that this will lead to less testing and exposure to full LVM use within the Fedora community.

He did not find much support on the list, though; most participants in the discussion seem interested in pushing forward and using the interesting features that btrfs has to offer. Lennart Poettering would like to take things further by splitting the installed drive into three subvolumes, one of which (holding the root filesystem) would be mounted read-only. This scheme would separate the system software from user files and protect the system from changes most of the time, but there would be no need to worry about filesystem sizing since btrfs can expand any of the subvolumes when needed.

That, of course, would be a significant change; having to remount the root filesystem for write access to install an update or make a configuration change could get old relatively quickly. But it could also improve the security of a running system and may be a good configuration for a number of environments. The ability to take snapshots of the root partition and roll the system back in case of trouble would be a nice added bonus.

The one other thing to be kept in mind is that btrfs, despite the speed with which it is maturing, will certainly have a surprise or two in store for its users still. Such is the nature of a new filesystem. But that is also the nature of free software: at some point widespread real-world testing is required to shake out the last round of bugs. Fedora seems like it could be a good place for this level of testing. Whether the ambitious Fedora 16 target will be met remains to be seen, but, if a btrfs default does not happen then, it can probably be expected soon thereafter.

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