> On May 16, 2025, at 10:22 AM, Mike Sun <m...@msun.io> wrote: > > The Cassandra docs > <https://cassandra.apache.org/doc/5.0/cassandra/managing/operating/repair.html> > advise: >> >> At a minimum, repair should be run often enough that the gc grace period >> never expires on unrepaired data. Otherwise, deleted data could reappear. >> With a default gc grace period of 10 days, repairing every node in your >> cluster at least once every 7 days will prevent this, while providing enough >> slack to allow for delays. > > I don't think repairing at least once every 7 days if gc_grace_seconds is 10 > days is adequate to guarantee no risk of data resurrection. > > I wrote this post to explain my reasoning: > https://msun.io/cassandra-scylla-repairs/ > <https://msun.io/cassandra-scylla-repairs/> > > Would appreciate any feedback, thanks! > Mike Sun
To summarize the blog for those who haven’t read it: Running repairs once every gc_grace_seconds is actually insufficient because it doesn’t account for the duration of the repair process itself and the specific timing of when data ranges (tokens) are repaired. A tombstone created for data just after its specific token was scanned by one repair can expire before the next repair cycle (which only begins gc_grace_seconds later) manages to reach and process that particular token. You need to complete the repair within the gc_grace_seconds window. Having repair run for 3 days would be a surprise. We can certainly adjust the wording, but the intent of that wording isn’t “start it every 7 days regardless of how often it runs”, it’s “finish it every 7 days” (successfully). Yes, it’s not enough to start the repair every 7 days, it needs to complete successfully between the time the tombstone is written and the expiration of gc_grace_seconds.
