I think "userspace COW" is the wrong approach here.
The entire idea of getting a point-in-time snapshot of something subject to rapid change is problematic. Your options boil down to (1) make a "snapshot" and save that (Redis's current approach) or (2) accept that point-in-time consistency might be impossible, and work around it.
I wonder whether it'd be possible to have two persistence strategies in Redis: "consistent" and "low-latency". "Consistent" would use the current fork(2) COW behavior, "low-latency" would do some kind of one-chunk-at-a-time block copy and amortize the latency spike over the entire operation, while having the overall effect of less of a "cliff" to latency.
The entire idea of getting a point-in-time snapshot of something subject to rapid change is problematic. Your options boil down to (1) make a "snapshot" and save that (Redis's current approach) or (2) accept that point-in-time consistency might be impossible, and work around it.
I wonder whether it'd be possible to have two persistence strategies in Redis: "consistent" and "low-latency". "Consistent" would use the current fork(2) COW behavior, "low-latency" would do some kind of one-chunk-at-a-time block copy and amortize the latency spike over the entire operation, while having the overall effect of less of a "cliff" to latency.