Cephalocon 2024

A Ceph cluster stretched across 3 zones faces a potential scenario where data loss can occur due to unforeseeable circumstances. An example of such a scenario is when we have 6 replicas spread across 3 datacenters with a min_size of 3 and the setup is intended to prevent I/O from happening when there is only 1 datacenter available, however, there is an edge case where a placement group (PG) becomes available due to a lack of safeguarding during the process of temporary PG mappings in order ensure data availability. This scenario poses a risk when the sole surviving data center accepts writes, and then suddenly the 2 unavailable data centers come back up. At the same time, the surviving data center suddenly goes down, which means we would have a data loss situation. To prevent such a scenario from happening, we created a solution that utilizes an existing feature in stretch mode that would restrict how we choose the OSDs that would go into the acting set of a PG. This talk will take a deep dive into how this feature is implemented in the latest Ceph upstream as well as other features that improve the user experience with stretch cluster in the latest Ceph upstream release.