关于long_waits列在V$BACKUP_ASYNC_IO中的疑问

Question

为什么总是有许多非零的long_waits值在V$BACKUP_ASYNC_IO中，这意味着等待O/S阻塞I/O完成？

注意:众所周知，async_io模式不能阻塞I/O，而sync_io模式可以两者兼而有之！

https://stackoverflow.com/questions/8416874/whats-the-differences-between-blocking-with-synchronous-nonblocking-and-asynch

https://stackoverflow.com/questions/2625493/asynchronous-vs-non-blocking

 IO_COUNT      READY SHORT_WAITS LONG_WAITS FILENAME                                                 TO_CHAR(CLOSE_TIME,        EPS
---------- ---------- ----------- ---------- -------------------------------------------------------- ------------------- ----------
        27         26           0          1 +DATA/cpemsdb/data_d-cpemsdb_ts-h_tzxx_s1_fno-389          11/27/2020 23:46:53     394202
        27         25           0          2 +DATA/cpemsdb/data_d-cpemsdb_ts-data_m_fno-198             11/27/2020 23:46:51     389805
         7          7           0          0 +DATA/cpemsdb/data_d-cpemsdb_ts-bbxx_fno-38                11/27/2020 23:46:33    2097152
      7682       7104           0        578 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_s_fno-676              11/27/2020 23:46:25    4145188
      7682       6905           0        777 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_p_fno-612              11/27/2020 23:46:21    4141990
      7682       7165           0        517 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_e_fno-515              11/27/2020 23:42:24    4266524
      7682       7166           0        516 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_a_fno-401              11/27/2020 23:42:18    4265394
      7682       7167           0        515 +DATA/cpemsdb/data_d-cpemsdb_ts-data_p_fno-316             11/27/2020 23:41:51    4276720
      7682       7166           0        516 +DATA/cpemsdb/data_d-cpemsdb_ts-data_out_fno-260           11/27/2020 23:41:25    4288106
      7682       7166           0        516 +DATA/cpemsdb/data_d-cpemsdb_ts-data_man_fno-203           11/27/2020 23:40:57    4301850
      7682       7047           0        635 +DATA/cpemsdb/data_d-cpemsdb_ts-data_a_fno-43              11/27/2020 23:40:14    4322052
     82537      76975           0       5562                                                          11/27/2020 22:22:57   37828972
      7682       7137           0        545 +DATA/cpemsdb/data_d-cpemsdb_ts-data_out_fno-781           11/27/2020 22:22:57   11422785
      5122       4773           0        349 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_arc_fno-439            11/27/2020 22:16:12    8910721
      4098       3821           0        277 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_e_fno-504              11/27/2020 22:10:47    8271483
      4098       3821           0        277 +DATA/cpemsdb/data_d-cpemsdb_ts-data_arc_fno-115           11/27/2020 22:10:45    8263525
      7682       6859           0        823 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_arc_fno-780            11/27/2020 21:44:07   28658590
     82537      76630           0       5907                                                          11/27/2020 21:44:07   44843779
      5122       4746           0        376 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_arc_fno-438            11/27/2020 21:40:36   23650701
      4098       3812           0        286 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_e_fno-498              11/27/2020 21:39:24   20773723
      4098       3813           0        285 +DATA/cpemsdb/data_d-cpemsdb_ts-data_arc_fno-97            11/27/2020 21:39:22   20698638
        27         25           0          2 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_yxbz_fno-689           11/27/2020 21:36:17   14979657
        27         26           0          1 +DATA/cpemsdb/data_d-cpemsdb_ts-data_ics_fno-193           11/27/2020 21:36:15   20971520
        27         25           0          2 +DATA/cpemsdb/data_d-cpemsdb_ts-hgqjd_fno-384              11/27/2020 21:36:15   20971520
      7682       7168           0        514 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_s_fno-674              11/27/2020 21:36:11    5206442
      7682       7168           0        514 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_out_fno-609            11/27/2020 21:36:10    5191338
      7682       7169           0        513 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_e_fno-501              11/27/2020 21:36:10    5175491
      7682       7168           0        514 +DATA/cpemsdb/data_d-cpemsdb_ts-idx_a_fno-399              11/27/2020 21:36:10    5157261
      7682       7169           0        513 +DATA/cpemsdb/data_d-cpemsdb_ts-data_p_fno-314             11/27/2020 21:36:06    5139980
      7682       7169           0        513 +DATA/cpemsdb/data_d-cpemsdb_ts-data_out_fno-258           11/27/2020 21:36:02    5125259
      7682       7168           0        514 +DATA/cpemsdb/data_d-cpemsdb_ts-data_man_fno-201           11/27/2020 21:35:57    5113056

Answer 1

来自https://docs.oracle.com/database/121/REFRN/GUID-1BF9B6DD-F1A0-49B8-AB8D-337FD361911D.htm#REFRN30018的文档

LONG_WAITS缓冲区无法立即可用的次数，并且只有在发出阻塞等待后才可用

缓冲区可能因任何原因而不可用。这可能会限制IO子系统或其他相关数据库资源的吞吐量。