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Sorry, I lost the previous thread on this. I apologize for the resulting incomplete reply. The issue that we’re having with Jewel, as David Turner mentioned, is that we can’t seem to throttle snap trimming sufficiently to prevent it from blocking I/O requests. On further investigation, I encountered osd_op_pq_max_tokens_per_priority,
which should be able to be used in conjunction with ‘osd_op_queue = wpq’ to govern the availability of queue positions for various operations using costs if I understand correctly. I’m testing with RBDs using 4MB objects, so in order to leave plenty of room
in the weighted priority queue for client I/O, I set osd_op_pq_max_tokens_per_priority to 64MB and osd_snap_trim_cost to 32MB+1. I figured this should essentially reserve 32MB in the queue for client I/O operations, which are prioritized higher and therefore
shouldn’t get blocked. I still see blocked I/O requests, and when I dump in-flight ops, they show ‘op must wait for map.’ I assume this means that what’s blocking the I/O requests at this point is all of the osdmap updates caused by snap trimming, and not the
actual snap trimming itself starving the ops of op threads. Hammer is able to mitigate this with osd_snap_trim_sleep by directly throttling snap trimming and therefore causing less frequent osdmap updates, but there doesn’t seem to be a good way to accomplish
the same thing with Jewel. First of all, am I understanding these settings correctly? If so, are there other settings that could potentially help here, or do we just need something like Sam already mentioned that can sort of reserve threads for client I/O requests?
Even then it seems like we might have issues if we can’t also throttle snap trimming. We delete a LOT of RBD snapshots on a daily basis, which we recognize is an extreme use case. Just wondering if there’s something else to try or if we need to start working
toward implementing something new ourselves to handle our use case better.
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