Tag Archives: btrfs

Debian-Live Rescue image with ZFS On Linux; Ditched btrfs

I’m a geek. I enjoy playing with different filesystems, version control systems, and, well, for that matter, radios.

I have lately started to worry about the risks of silent data corruption, and as such, looked to switch my personal systems to either ZFS or btrfs, both of which offer built-in checksumming of all data and metadata. I initially opted for btrfs, because of its tighter integration into the Linux kernel and ability to shrink an existing btrfs filesystem.

However, as I wrote last month, that experiment was not a success. I had too many serious performance regressions and one too many kernel panics and decided it wasn’t worth it. And that the SuSE people got it wrong, deeply wrong, when they declared btrfs ready for production. I never lost any data, to its credit. But it simply reduces uptime too much.

That left ZFS. Before I build a system, I always want to make sure I can repair it. So I started with the Debian Live rescue image, and added the zfsonlinux.org repository to it, along with some key packages to enable the ZFS kernel modules, GRUB support, and initramfs support. The resulting image is described, and can be downloaded from, my ZFS Rescue Disc wiki page, which also has a link to my source tree on github.

In future blog posts in the series, I will describe the process of converting existing Debian installations to use ZFS, of getting them to boot from ZFS, some bugs I encountered along the way, and some surprising performance regressions in ZFS compared to ext4 and btrfs.

Results with btrfs and zfs

The recent news that openSUSE considers btrfs safe for users prompted me to consider using it. And indeed I did. I was already familiar with zfs, so considered this a good opportunity to experiment with btrfs.

btrfs makes an intriguing filesystem for all sorts of workloads. The benefits of btrfs and zfs are well-documented elsewhere. There are a number of features btrfs has that zfs lacks. For instance:

  • The ability to shrink a device that’s a member of a filesystem/pool
  • The ability to remove a device from a filesystem/pool entirely, assuming enough free space exists elsewhere for its data to be moved over.
  • Asynchronous deduplication that imposes neither a synchronous performance hit nor a heavy RAM burden
  • Copy-on-write copies down to the individual file level with cp --reflink
  • Live conversion of data between different profiles (single, dup, RAID0, RAID1, etc)
  • Live conversion between on-the-fly compression methods, including none at all
  • Numerous SSD optimizations, including alignment and both synchronous and asynchronous TRIM options
  • Proper integration with the VM subsystem
  • Proper support across the many Linux architectures, including 32-bit ones (zfs is currently only flagged stable on amd64)
  • Does not require excessive amounts of RAM

The feature set of ZFS that btrfs lacks is well-documented elsewhere, but there are a few odd btrfs missteps:

  • There is no way to see how much space subvolume/filesystem is using without turning on quotas. Even then, it is cumbersome and not reported with df like it should be.
  • When a maxmium size for a subvolume is set via a quota, it is not reported via df; applications have no idea when they are about to hit the maximum size of a filesystem.

btrfs would be fine if it worked reliably. I should say at the outset that I have never lost any data due to it, but it has caused enough kernel panics that I’ve lost count. I several times had a file that produced a panic when I tried to delete it, several times when it took more than 12 hours to unmount a btrfs filesystem, behaviors where hardlink-heavy workloads take days longer to complete than on zfs or ext4, and that’s just the ones I wrote about. I tried to use btrfs balance to change the metadata allocation on the filesystem, and never did get it to complete; it seemed to go into an endless I/O pattern after the first 1GB of metadata and never got past that. I didn’t bother trying the live migration of data from one disk to another on this filesystem.

I wanted btrfs to work. I really, really did. But I just can’t see it working. I tried it on my laptop, but had to turn of CoW on my virtual machine’s disk because of the rm bug. I tried it on my backup devices, but it was unusable there due to being so slow. (Also, the hardlink behavior is broken by default and requires btrfstune -r. Yipe.)

At this point, I don’t think it is really all that worth bothering with. I think the SuSE decision is misguided and ill-informed. btrfs will be an awesome filesystem. I am quite sure it will, and will in time probably displace zfs as the most advanced filesystem out there. But that time is not yet here.

In the meantime, I’m going to build a Debian Live Rescue CD with zfsonlinux on it. Because I don’t ever set up a system I can’t repair.