PGs in Ceph maintain a log of writes to allow speedy recovery of data. Instead of scanning all of the objects to see what is missing on each osd, we can examine the pg log to see which objects we need to recover. See Log Based PG for more details on this process.
Until now, this recovery process was synchronous - it blocked writes to an object until it was recovered. In contrast, backfill could allow writes to proceed (assuming enough up-to-date copies of the data were available) by temporarily assigning a different acting set, and backfilling an OSD outside of the acting set. In some circumstances, this ends up being significantly better for availability, e.g. if the pg log contains 3000 writes to different objects. Recovering several megabytes of an object (or even worse, several megabytes of omap keys, like rgw bucket indexes) can drastically increase latency for a small update, and combined with requests spread across many degraded objects it is a recipe for slow requests.
To avoid this, we can perform recovery in the background on an OSD out of the acting set, similar to backfill, but still using the PG log to determine what needs recovery. This is known as asynchronous recovery.
Exactly when we perform asynchronous recovery instead of synchronous recovery is not a clear-cut threshold. There are a few criteria which need to be met for asynchronous recovery:
try to keep min_size replicas available
use the approximate magnitude of the difference in length of logs combined with historical missing objects as the cost of recovery
use the parameter osd_async_recovery_min_cost to determine when asynchronous recovery is appropriate
With the existing peering process, when we choose the acting set we have not fetched the pg log from each peer, we have only the bounds of it and other metadata from their pg_info_t. It would be more expensive to fetch and examine every log at this point, so we only consider an approximate check for log length for now. In Nautilus, we improved the accounting of missing objects, so post nautilus, this information is also used to determine the cost of recovery.
While async recovery is occurring, writes on members of the acting set may proceed, but we need to send their log entries to the async recovery targets (just like we do for backfill osds) so that they can completely catch up.