Ultimately, they selected CockroachDB, as described in "RFD 110: CockroachDB for the control plane database" [1]. Now, published just yesterday, "RDD 508: Whither CockroachDB?" [2] discusses what to do in light of Cockroach going proprietary.

[1]: https://rfd.shared.oxide.computer/rfd/0110 [2]: https://rfd.shared.oxide.computer/rfd/0508