Hi- I’d like to restart the discussion in this thread: http://marc.info/?l=linux-nfs&m=141348840527766&w=2 It seems to me there are two main points: 1. Is bi-directional RPC on RPC/RDMA transports desirable? 2. Is a secondary backchannel-only transport adequate and reliable? I’ll try to summarize the current thinking. Question 1: The main reason to plumb bi-RPC into RPC/RDMA is that no changes to the NFSv4.1 client upper layers would be needed. I think we also agree that: - There is no performance benefit. CB operations typically lack significant payload, are infrequent, and can be long-running. - There is no need to penetrate firewalls. Firewall compatibility was the original motivation for single-transport NFSv4.1 operation. Firewalls are not typically found in RDMA-native environments. - There is no requirement in RFC 5661 for the forward channel transport to support bi-directional RPC. Backchannel capability is detected via the CREATE_SESSION operation. - TCP connectivity will always be available wherever NFS/RDMA is deployed. For NFS/RDMA operation, IP address to GUID mapping must be provided by the transport layer, below RPC/RDMA. - To handle large payloads (possibly required by certain pNFS CB operations), an NFSv4.1 client would need to handle RDMA_NOMSG type calls over the backchannel. This would require the client to perform RDMA READ and WRITE operations against the server (the opposite of what happens in the forward channel). There is some interest in prototyping an RPC/RDMA transport that is capable of bi-directional RPC. A prototype would help us determine whether there are subtle problems that make bi-RPC impossible for RPC/RDMA, and identify any spec gaps that need to be addressed. Because of the development cost and lack of perceptible benefits, a prototype has not been attempted so far. Would it be productive for a bi-capable RPC/RDMA transport prototype to be pursued in Linux? Question 2: The Solaris client and server already implement a sidecar TCP backchannel for NFSv4.1. This is something that can be tested. Further, I think we agree that: - Servers are required to support a separate backchannel and forward channel transport, and both sides can detect what is supported with CREATE_SESSION. However, there are no existing implementations that have deployed this kind of logic widely. - The addition of a separate backchannel-only connection is considered session trunking, which is regarded as potentially hazardous. We haven’t identified exactly what the hazards might be when the second connection handles only backchannel activity. - Although there are few or no server changes required to support a secondary backchannel, clients would have to be modified to establish this connection when one or both sides do not support a backchannel on the main transport and the server asserts the SEQ4_STATUS_CB_PATH_DOWN flag. - We have some confidence that creation of the second backchannel- only connection followed by BIND_CONN_TO_SESSION appears to be adequate and robust. However, the salient recovery edge conditions when a secondary backchannel transport is being used still need to be identified. What further investigation is needed to be confident that the sidecar solution is adequate and appropriate? -- 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
