Ceph has the ability to prioritize certain operations over others, with the idea that the client I/Os should have precedence over the recovery, scrubbing, and snapshot trimming IO. These priorities are controlled by the following configuration options:

osd client op priority
osd recovery op priority
osd scrub priority
osd snap trim priority

Here, the higher the value, the higher priority. The default values work fairly well, and there shouldn't be much requirement to change them. But there can be some benefit in lowering the priority of scrub and recovery operations to limit their impact on the client I/O. It's important to understand that Ceph can only prioritize the I/O in the sections of the I/O path that it controls. Therefore, tuning the disk queue lengths in the previous section may be needed to get maximum benefit.