Thanks for the question! Joran from TigerBeetle here.

The research in question is the 2018 paper from UW-Madison, “Protocol-Aware Recovery for Consensus-Based Storage” (PAR) [0] by Ram Alagappan, Aishwarya Ganesan, as well as Remzi and Andrea Arpaci-Dusseau (who you may recognize as authors of OSTEP).

PAR won best paper at FAST '18 for showing that a single disk sector fault, in the write-ahead log (WAL) of a single replica, could propagate through the distributed RAFT or MultiPaxos consensus protocol, to cause global cluster data loss.

This was counter-intuitive at the time, because PAR showed that the redundancy of these consensus and replication protocols did not in fact always imply fault-tolerance, as had previously been assumed.

The reason is, and we cover this in depth in our recent QCon London talk [1], but it was assumed that checksums alone would be sufficient to detect and recover from storage faults.

However, while checksums can be used under the “Crash Consistency Model” to solve consistency through power loss, PAR showed that checksums are not sufficient to be able to distinguish between a torn write at the end of the (uncommitted) WAL caused by power loss, and a torn write in the middle of the (committed) WAL caused by bitrot.

What you tend to find is that the WALs for many of these protocols will truncate the WAL at the first sign of a checksum mismatch, conflating the mismatch with power loss when it might be bitort, and thereby truncating committed transactions, and undermining quorum votes in the Raft or MultiPaxos implementations.

RAID solutions don't always help here, either. See "Parity Lost and Parity Regained" [2] for more details. ZRAID is better here, and ZFS is a huge inspiration, but with local redundancy under ZFS you're still not leveraging the global redundancy of the consensus protocol as well as you could be.

To summarize PAR:

There are fundamental design changes to both the global consensus protocol and the local storage engine that would need to be made, if the storage fault model of PAR (and TigerBeetle) is to be solved correctly.

Furthermore, few simulators even test for these kinds of storage faults. For example, misdirected I/O, where the disk writes or reads to or from the wrong location of disk, which may yet have a valid checksum.

However, this is important, because disks fail in the real world. A single disk has on the order of a 0.5-1% chance of corruption in a 2 year period [3]. For example, a 5 node cluster has a 2.5-5% chance of a single disk sector fault, which again in terms of PAR can lead to global cluster data loss.

On the other hand, memory (or even CPU) faults, assuming ECC are not in the same order of magnitude probability, and therefore TigerBeetle's memory fault model is to require ECC memory.

But, again, to be crystal clear, checksums alone are not sufficient to solve the consensus corruption issue. The fix requires protocol changes at the design level, for the consensus protocol to be made storage fault-aware.

Thanks for the question and happy to answer more!

[0] “Protocol-Aware Recovery for Consensus-Based Storage” https://www.usenix.org/conference/fast18/presentation/alagap...

[1] “A New Era for Database Design” (we also dive into the research surrounding Fsyncgate, looking into the latent correctness issues that remain) https://www.youtube.com/watch?v=_jfOk4L7CiY

[2] “Parity Lost and Parity Regained” https://www.usenix.org/conference/fast-08/parity-lost-and-pa...

[3] “An Analysis of Data Corruption in the Storage Stack” https://www.cs.toronto.edu/~bianca/papers/fast08.pdf