Azure Cosmos DB中会话一致性更新慢、强一致性更新快的原因？

我使用YCSB工具对Azure Cosmos DB的强一致性和会话一致性级别开展基准测试，测试配置如下：

• 区域配置：美国东部为单写入区域，美国西部为副本读取区域
• 容器配置：吞吐量手动设置为6000 RUs
• 客户端部署：2台位于美国东部区域的VM，每台客户端使用50个线程
• 测试数据集：200000条1KB大小的记录
• 测试负载：读-修改-写工作负载（对应YCSB的workloadf）

测试结果 #

强一致性（STRONG） #

[OVERALL], RunTime(ms), 1802004  
[OVERALL], Throughput(ops/sec), 166.4813174665539  
[READ], Operations, 300000  
[READ], AverageLatency(us), 201056.00650333334  
[READ], MinLatency(us), 1581  
[READ], MaxLatency(us), 2156543  
[READ], 95thPercentileLatency(us), 603135  
[READ], 99thPercentileLatency(us), 1172479  
[READ], Return=OK, 300000  
[READ-MODIFY-WRITE], Operations, 300000  
[READ-MODIFY-WRITE], AverageLatency(us), 298607.42890666664  
[READ-MODIFY-WRITE], MinLatency(us), 78080  
[READ-MODIFY-WRITE], MaxLatency(us), 2306047  
[READ-MODIFY-WRITE], 95thPercentileLatency(us), 717823  
[READ-MODIFY-WRITE], 99thPercentileLatency(us), 1265663  
[UPDATE], Operations, 300000  
[UPDATE], AverageLatency(us), 97547.07946666666  
[UPDATE], MinLatency(us), 75776  
[UPDATE], MaxLatency(us), 1281023  
[UPDATE], 95thPercentileLatency(us), 121151  
[UPDATE], 99thPercentileLatency(us), 128063  
[UPDATE], Return=OK, 300000  

会话一致性（SESSION） #

[OVERALL], RunTime(ms), 1350937  
[OVERALL], Throughput(ops/sec), 222.0680905179146  
[READ], Operations, 300000  
[READ], AverageLatency(us), 28163.643546666666  
[READ], MinLatency(us), 1426  
[READ], MaxLatency(us), 1573887  
[READ], 95thPercentileLatency(us), 101503  
[READ], 99thPercentileLatency(us), 508415  
[READ], Return=OK, 300000  
[READ-MODIFY-WRITE], Operations, 300000  
[READ-MODIFY-WRITE], AverageLatency(us), 221261.26997333334  
[READ-MODIFY-WRITE], MinLatency(us), 85760  
[READ-MODIFY-WRITE], MaxLatency(us), 3274751  
[READ-MODIFY-WRITE], 95thPercentileLatency(us), 648703  
[READ-MODIFY-WRITE], 99thPercentileLatency(us), 1141759  
[UPDATE], Operations, 300000  
[UPDATE], AverageLatency(us), 193093.33994666667  
[UPDATE], MinLatency(us), 83712  
[UPDATE], MaxLatency(us), 3110911  
[UPDATE], 95thPercentileLatency(us), 626175  
[UPDATE], 99thPercentileLatency(us), 1129471  
[UPDATE], Return=OK, 300000  

结果疑问 #

测试结果存在超出预期的现象：

• 会话一致性下更新操作的平均延迟（193093us）远高于强一致性（97547us），强一致性下更新更快
• 读取操作则相反，会话一致性的平均读取延迟（28163us）远低于强一致性（201056us）
• 读-修改-写操作的结果符合预期，会话一致性下平均延迟略低于强一致性

内容的提问来源于stack exchange，提问作者aman_kumar