Varuna: Enabling Failure-Type Aware RDMA Failover

2026-03-29

AI Summary

This work addresses the inefficiencies of existing fault-tolerance mechanisms for RDMA link failures, which uniformly retransmit all in-flight requests upon connection disruption, leading to wasted bandwidth, semantic errors, and high recovery overhead. The authors propose a failure-type-aware recovery mechanism that leverages a lightweight completion log to track the execution status of each request, thereby distinguishing between already executed and unexecuted operations. By selectively retransmitting only necessary requests and restoring their results, this approach achieves precise, execution-state-based retransmission for the first time. It prevents redundant execution of non-idempotent operations, ensures transactional consistency, and eliminates the need for connection reestablishment. Experimental results demonstrate negligible memory overhead, a steady-state latency penalty of merely 0.6–10%, and a 65% reduction in recovery retransmission time.

Abstract

RDMA link failures can render connections temporarily unavailable, causing both performance degradation and significant recovery overhead. To tolerate such failures, production datacenters assign each primary link with a standby link and, upon failure, uniformly retransmit all in-flight RDMA request over the backup path. However, we observe that such blanket retransmission is unnecessary. In-flight requests can be split into pre-failure and post-failure categories depending on whether the responder has already executed. Retransmitting post-failure requests is not only redundant (consuming bandwidth), but also incorrect for non-idempotent operations, where duplicate execution can violate application semantics. We present Varuna, a failure-type-aware RDMA recovery mechanism that enables correct retransmission and us-level failover. Varuna piggybacks a lightweight completion log on every RDMA operation; after a link failure, this log deterministically reveals which in-flight requests were executed (post-failure) and which were lost (pre-failure). Varuna then retransmits only the pre-failure subset and fetches/recovers the return values for post-failure requests. Evaluated using synthetic microbenchmarks and end-to-end RDMA TPC-C transactions, Varuna incurs only 0.6-10% steady-state latency overhead in realistic applications, eliminates 65% of recovery retransmission time, preserves transactional consistency, and introduces zero connectivity rebuild overhead and negligible memory overhead during RDMA failover.

Problem

• RDMA failover
• failure-type awareness
• non-idempotent operations
• retransmission redundancy
• transactional consistency

Innovation

• failure-type-aware
• RDMA failover
• completion log
• non-idempotent operations
• microsecond-level recovery