I'm trying to better understand rbd features but I have only found the 
information on the RBD page, is there any further RBD feature information and 
implementation?

also I would like to know about journaling feature, it seems to destroy rbd 
performance:
Without journaling feature:
rbd bench --io-type write nmve/test --io-size 4M
bench  type write io_size 4194304 io_threads 16 bytes 1073741824 pattern 
sequential
  SEC       OPS   OPS/SEC   BYTES/SEC
    1       165    167.70  703388666.65
elapsed:     1  ops:      256  ops/sec:   147.34  bytes/sec: 617979315.69

With journaling feature:
rbd bench --io-type write nmve/test --io-size 4M
bench  type write io_size 4194304 io_threads 16 bytes 1073741824 pattern 
sequential
  SEC       OPS   OPS/SEC   BYTES/SEC
    1        39     40.07  168083012.60
    2        75     39.57  165958196.86
    3       111     39.29  164800587.00
    4       146     40.45  169644582.55
    5       181     38.99  163537471.86
    6       215     35.93  150720311.94
    7       240     33.29  139648167.70
elapsed:     7  ops:      256  ops/sec:    34.36  bytes/sec: 144134536.70

What is the reason of that behavior? What journaling does?


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