We look at the amazing features in ZFS and btrfs—and why you need them.
The MP3 that startled you with a violent CHIRP!, and you wondered if it had always done that? No, it probably hadn't—blame bitrot. The video with a bright green block in one corner followed by several seconds of weird rainbowy blocky stuff before it cleared up again? Bitrot.
The worst thing is that backups won't save you from bitrot. The next backup will cheerfully back up the corrupted data, replacing your last good backup with the bad one. Before long, you'll have rotated through all of your backups (if you even have multiple backups), and the uncorrupted original is now gone for good.
Contrary to popular belief, conventional RAID won't help with bitrot, either. "But my raid5 array has parity and can reconstruct the missing data!" you might say. That only works if a drive completely and cleanly fails. If the drive instead starts spewing corrupted data, the array may or may not notice the corruption (most arrays don't check parity by default on every read). Even if it does notice... all the array knows is that something in the stripe is bad; it has no way of knowing which drive returned bad data—and therefore which one to rebuild from parity (or whether the parity block itself was corrupt).
What might save your data, however, is a "next-gen" filesystem.
As a test, I set up a virtual machine with six drives. One has the operating system on it, two are configured as a simple btrfs-raid1 mirror, and the remaining three are set up as a conventional raid5. I saved Finn's picture on both the btrfs-raid1 mirror and the conventional raid5 array, and then I took the whole system offline and flipped a single bit—yes, just a single bit from 0 to 1—in the JPG file saved on each array. Here's the result:The raid5 array didn't notice or didn't care about the flipped bit in Finn's picture any more than a standard single disk would. The next-gen btrfs-raid1 system, however, immediately caught and corrected the problem. The results are pretty obvious. If you care about your data, you want a next-gen filesystem. Here, we'll examine two: the older ZFS and the more recent btrfs.
if you accept my definition, "next-generation" currently means "fifth generation." It's defined by an entire set of features: built-in volume management, per-block checksumming, self-healing redundant arrays, atomic COW snapshots, asynchronous replication, and far-future scalability.
