On Apr 25, 2013, at 9:49 AM, bfields@xxxxxxxxxxxx wrote: > On Thu, Apr 25, 2013 at 01:30:58PM +0000, Myklebust, Trond wrote: >> On Thu, 2013-04-25 at 09:29 -0400, bfields@xxxxxxxxxxxx wrote: >> >>> My position is that we simply have no idea what order of magnitude even >>> delay should be. And that in such a situation exponential backoff such >>> as implemented in the synchronous case seems the reasonable default as >>> it guarantees at worst doubling the delay while still bounding the >>> long-term average frequency of retries. >> >> So we start with a 15 second delay, and then go to 60 seconds? > > I agree that a server should normally be doing the wait on its own if > the wait would be on the order of an rpc round trip. > > So I'd be inclined to start with a delay that was an order of magnitude > or two more than a round trip. > > And I'd expect NFS isn't common on networks with 1-second latencies. > > So the 1/10 second we're using in the synchronous case sounds closer to > the right ballpark to me. The RPC layer already keeps RPC round trip statistics, so the client doesn't have to guess with a "one size fits all" number. I'm all for keeping client recovery time short. But after following this argument, I think 10xRTT is crazy short. Aggressive retransmits can lead to data corruption, and RTT on a fast server is going to be on the order of a millisecond. And what about RDMA, where RTT is about 20usecs? A better answer might be to start at one second then exponentially back off to the minimum of 0.25x the lease time and 0.25x the RPC retransmit time out. -- Chuck Lever chuck[dot]lever[at]oracle[dot]com -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
