Practical Quick File Server Migration

Abstract

Regular file server upgrades are indispensable to improve performance, robustness, and power consumption. In upgrading file servers, it is crucial to quickly migrate file-sharing services between heterogeneous servers with little downtime while minimizing performance interference. We present a practical quick file server migration scheme based on the postcopy approach that defers file copy until after switching servers. This scheme can (1) reduce downtime with on-demand file migration, (2) avoid performance interference using background migration, and (3) support heterogeneous servers with stub-based file management. We discuss several practical issues, such as intermittent crawling and traversal strategy, and present the solutions in our scheme. We also address several protocol-specific issues to achieve a smooth migration. This scheme is good enough to be adopted in production systems, as it has been demonstrated for several years in real operational environments. The performance evaluation demonstrates that the downtime is less than 3 seconds, and the first file access after switching servers does not cause a timeout in the default timeout settings; it takes less than 10 seconds in most cases and up to 84.55 seconds even in a large directory tree with a depth of 16 and a width of 1,000. Although the total migration time is approximately 3 times longer than the traditional precopy approach that copies all files in advance, our scheme allows the clients to keep accessing files with acceptable overhead. We also show that appropriate selection of traversal strategy reduces tail latency by 88%, and the overhead after the migration is negligible.