Flush tasks a lot of time with no other operations after the db created

[del-zhenwu]

Please state your issue using the following template and, most importantly, in English.

Describe the bug

2021-07-01 15:25:41,036 - milvus_benchmark.client:49 - DEBUG - Milvus load_collection run in 0.04s
2021-07-01 15:25:41,080 - milvus_benchmark.client:49 - DEBUG - Milvus load_collection run in 0.04s
2021-07-01 15:25:41,086 - locust.stats_logger:720 - INFO -  Name                                                          # reqs      # fails  |     Avg     Min     Max  Median  |   req/s failures/s
2021-07-01 15:25:41,092 - locust.stats_logger:722 - INFO - --------------------------------------------------------------------------------------------------------------------------------------------
2021-07-01 15:25:41,098 - locust.stats_logger:725 - INFO -  grpc flush                                                       105     0(0.00%)  |     580     528     711     580  |    1.40    0.00
2021-07-01 15:25:41,104 - locust.stats_logger:725 - INFO -  grpc load_collection                                             135     0(0.00%)  |      47      35      71      47  |    2.90    0.00
2021-07-01 15:25:41,110 - locust.stats_logger:726 - INFO - --------------------------------------------------------------------------------------------------------------------------------------------
2021-07-01 15:25:41,116 - locust.stats_logger:727 - INFO -  Aggregated                                                       240     0(0.00%)  |     280      35     711      53  |    4.30    0.00
2021-07-01 15:25:41,122 - locust.stats_logger:728 - INFO - 

Steps/Code to reproduce behavior

1. create
2. insert
3. flush
4. Do loop flush, The flush operation spend too much time.

Expected behavior
A clear and concise description of what you expected to happen.

Method of installation

• Docker/cpu
• Docker/gpu
• Build from source

Environment details

• Hardware/Software conditions (OS, CPU, GPU, Memory)

• Milvus version (master or released version)

Configuration file
Settings you made in server_config.yaml or milvus.yaml

paste-file-content-here

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

[del-zhenwu]

/kind enhancement

[XuanYang-cn]

/assign

[stale]

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