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  1. Core Server
  2. SERVER-7973

Mongo should be able to reply to client requests for data in RAM while flushing to disk

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 2.5.5
    • Component/s: None
    • Labels:
      None
    • Operating System:
      ALL
    • Case:

      Description

      When MongoDB flushes to disk, the rate of inserts fall considerably with at least a 30% reduction.

      All the data is in memory and there are no page faults.

      The issues persists despite the o/s being highly modified with recommended MongoDB production settings - noatime, ext4, high ulimit, journal on separate disk,ssd and low readahead.

      mongostat snippet below showing the drop in query rates when a flush occurs:

      insert  query update delete getmore command flushes mapped  vsize    res non-mapped faults  locked db idx miss %     qr|qw   ar|aw  netIn netOut  conn    set repl       time 
          73   6713   6715      0    4859    6806       0  6.47g  13.6g  4.41g      7.12g      0 test:56.7%          0       1|0     0|1    31m    93m    50 rs1  PRI   15:00:00 
          54   4392   4381      0    3320    4431       0  6.47g  13.6g  4.42g      7.12g      0 test:58.6%          0      39|1     0|1    20m    60m    50 rs1  PRI   15:00:01 
          64   6078   6113      0    4569    6183       1  6.47g  13.6g   4.4g      7.12g      0 test:67.4%          0       0|0     0|0    29m    84m    50 rs1  PRI   15:00:02 
          65   5967   5954      0    4549    6020       0  6.47g  13.6g  4.38g      7.12g      0 test:52.3%          0       0|0     0|0    28m    82m    50 rs1  PRI   15:00:03 
          58   6192   6177      0    4748    6227       0  6.47g  13.6g   4.4g      7.12g      0 test:53.2%          0      39|0     0|1    29m    85m    50 rs1  PRI   15:00:04 
          65   6212   6246      0    4704    6321       0  6.47g  13.6g  4.38g      7.12g      0 test:53.9%          0       0|0     0|0    29m    86m    50 rs1  PRI   15:00:05 
          66   6355   6348      0    4957    6412       0  6.47g  13.6g  4.41g      7.12g      0 test:54.8%          0       1|0     0|1    30m    88m    50 rs1  PRI   15:00:06 
          61   6504   6487      0    4920    6539       0  6.47g  13.6g  4.39g      7.12g      0 test:55.3%          0      39|0     0|1    30m    90m    50 rs1  PRI   15:00:07 
          63   6654   6683      0    5006    6756       0  6.47g  13.6g  4.37g      7.12g      0 test:57.6%          0       0|0     0|0    31m    92m    50 rs1  PRI   15:00:08 
          65   6754   6746      0    4928    6811       0  6.47g  13.6g   4.4g      7.12g      0 test:58.0%          0       0|0     0|1    31m    93m    50 rs1  PRI   15:00:09 
      insert  query update delete getmore command flushes mapped  vsize    res non-mapped faults  locked db idx miss %     qr|qw   ar|aw  netIn netOut  conn    set repl       time 
          46   6166   6154      0    4524    6187       0  6.47g  13.6g  4.38g      7.12g      0 test:51.5%          0      39|0     0|1    29m    85m    50 rs1  PRI   15:00:10 
          40   3658   3679      0    2882    3734       1  6.47g  13.6g  4.38g      7.12g      0 test:43.8%          0       0|0     0|1    17m    51m    50 rs1  PRI   15:00:11 
          53   5821   5821      0    4196    5872       0  6.47g  13.6g  4.39g      7.12g      0 test:50.7%          0       4|0     1|1    27m    80m    50 rs1  PRI   15:00:12 
          64   6370   6366      0    4694    6432       0  6.47g  13.6g  4.38g      7.12g      0 test:55.0%          0       0|0     0|0    30m    88m    50 rs1  PRI   15:00:13 
          70   6332   6332      0    4794    6403       0  6.47g  13.6g  4.41g      7.12g      0 test:55.0%          0       0|0     0|1    30m    87m    50 rs1  PRI   15:00:14 
          66   6219   6209      0    4576    6265       0  6.47g  13.6g  4.39g      7.12g      0 test:53.7%          0      39|0     0|1    29m    86m    50 rs1  PRI   15:00:15 
          75   6151   6172      0    4823    6257       0  6.47g  13.6g  4.37g      7.12g      0 test:53.6%          0       0|0     0|1    29m    85m    50 rs1  PRI   15:00:16 
          65   6189   6189      0    4697    6255       0  6.47g  13.6g  4.39g      7.12g      0 test:53.3%          0       0|0     0|1    29m    85m    50 rs1  PRI   15:00:17 
          68   5998   5982      0    4509    6040       0  6.47g  13.6g  4.38g      7.12g      0 test:52.2%          0      39|0     0|1    28m    83m    50 rs1  PRI   15:00:18 
          62   5975   6001      0    4395    6074       0  6.47g  13.6g   4.4g      7.12g      0 test:54.9%          0       0|0     0|1    28m    83m    50 rs1  PRI   15:00:19 
      insert  query update delete getmore command flushes mapped  vsize    res non-mapped faults  locked db idx miss %     qr|qw   ar|aw  netIn netOut  conn    set repl       time 
          60   6213   6208      0    4744    6268       0  6.47g  13.6g  4.39g      7.12g      0 test:54.0%          0       1|0     0|1    29m    86m    50 rs1  PRI   15:00:20 
          43   3738   3732      0    2931    3776       1  6.47g  13.6g  4.38g      7.12g      0 test:37.9%          0      10|0     0|1    17m    51m    50 rs1  PRI   15:00:21 
          71   6220   6231      0    4783    6302       0  6.47g  13.6g  4.41g      7.12g      0 test:53.5%          0       1|0     1|0    29m    86m    50 rs1  PRI   15:00:22 
          70   6073   6073      0    4737    6144       0  6.47g  13.6g  4.39g      7.12g      0 test:59.9%          0       8|0     0|1    28m    84m    50 rs1  PRI   15:00:23 
          57   6608   6619      0    4928    6675       0  6.47g  13.6g  4.37g      7.12g      0 test:58.2%          0       0|0     0|1    31m    91m    50 rs1  PRI   15:00:24 
          70   6749   6742      0    5221    6814       0  6.47g  13.6g  4.38g      7.12g      0 test:58.2%          0       1|0     0|1    31m    93m    50 rs1  PRI   15:00:25 
          59   6423   6428      0    4897    6487       0  6.47g  13.6g  4.41g      7.12g      0 test:55.5%          0       6|1     1|1    30m    89m    50 rs1  PRI   15:00:26 
          72   6294   6306      0    4729    6379       0  6.47g  13.6g  4.38g      7.12g      0 test:57.5%          0       0|0     0|0    29m    87m    50 rs1  PRI   15:00:27 
          59   6706   6699      0    5002    6756       0  6.47g  13.6g   4.4g      7.12g      0 test:56.6%          0       0|0     0|0    31m    92m    50 rs1  PRI   15:00:28 
          61   6678   6682      0    4900    6743       0  6.47g  13.6g  4.39g      7.12g      0 test:57.6%          0       1|0     1|0    31m    92m    50 rs1  PRI   15:00:29 

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