Reading through their VLDB paper , Comdb2 appears to be a moderately scalable (up to dozens of nodes (?)) RDBMS with a strong emphasis on consistency and availability. Benchmarks show numbers comparable to Percona XtraDB (MySQL with a different storage engine), at ~2,000 writes/s and 2,000,000 reads/s against a 6 node server cluster. High availability and global sequencing is provided by using GPS clocks, similar to Spanner/Truetime.
Schema changes happen lazily, with old rows being rewritten on the next update, and a background job doing bulk rewriting.
Scalability: "While reads scale in a nearly linear manner as cluster size increases, writes have a more conservative level of scaling with an absolute limit. The offloading of portions of work (essentially all the WHERE clause predicate evaluations) across the cluster does help scale writes beyond what a single machine can do. Ultimately, this architecture does saturate on a single machine’s ability to process the low level bplogs."
This doesn't provide the horizontal scaling that Spanner does, CockroachDB aims at, or FoundationDB presumably has.