Re: Large latency with bcache for Ceph OSD

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Ping.

I'm confused on the SYNC I/O on bcache. why SYNC I/O must be writen back for persistent cache?  It can cause some latency.

@Coly, can you give help me to explain why bcache handle O_SYNC like this.?


On 2021/2/18 下午3:56, Norman.Kern wrote:
Hi guys,

I am testing ceph with bcache, I found some I/O with O_SYNC writeback to HDD, which caused large latency on HDD, I trace the I/O with iosnoop:

./iosnoop  -Q -ts -d '8,192

Tracing block I/O for 1 seconds (buffered)...
STARTs          ENDs            COMM         PID    TYPE DEV BLOCK        BYTES     LATms

1809296.292350  1809296.319052  tp_osd_tp    22191  R    8,192 4578940240   16384     26.70 1809296.292330  1809296.320974  tp_osd_tp    22191  R    8,192 4577938704   16384     28.64 1809296.292614  1809296.323292  tp_osd_tp    22191  R    8,192 4600404304   16384     30.68 1809296.292353  1809296.325300  tp_osd_tp    22191  R    8,192 4578343088   16384     32.95 1809296.292340  1809296.328013  tp_osd_tp    22191  R    8,192 4578055472   16384     35.67 1809296.292606  1809296.330518  tp_osd_tp    22191  R    8,192 4578581648   16384     37.91 1809295.169266  1809296.334041  bstore_kv_fi 17266  WS   8,192 4244996360   4096    1164.78 1809296.292618  1809296.336349  tp_osd_tp    22191  R    8,192 4602631760   16384     43.73 1809296.292618  1809296.338812  tp_osd_tp    22191  R    8,192 4602632976   16384     46.19 1809296.030103  1809296.342780  tp_osd_tp    22180  WS   8,192 4741276048   131072   312.68 1809296.292347  1809296.345045  tp_osd_tp    22191  R    8,192 4609037872   16384     52.70 1809296.292620  1809296.345109  tp_osd_tp    22191  R    8,192 4609037904   16384     52.49 1809296.292612  1809296.347251  tp_osd_tp    22191  R    8,192 4578937616   16384     54.64 1809296.292621  1809296.351136  tp_osd_tp    22191  R    8,192 4612654992   16384     58.51 1809296.292341  1809296.353428  tp_osd_tp    22191  R    8,192 4578220656   16384     61.09 1809296.292342  1809296.353864  tp_osd_tp    22191  R    8,192 4578220880   16384     61.52 1809295.167650  1809296.358510  bstore_kv_fi 17266  WS   8,192 4923695960   4096    1190.86 1809296.292347  1809296.361885  tp_osd_tp    22191  R    8,192 4607437136   16384     69.54 1809296.029363  1809296.367313  tp_osd_tp    22180  WS   8,192 4739824400   98304    337.95 1809296.292349  1809296.370245  tp_osd_tp    22191  R    8,192 4591379888   16384     77.90 1809296.292348  1809296.376273  tp_osd_tp    22191  R    8,192 4591289552   16384     83.92 1809296.292353  1809296.378659  tp_osd_tp    22191  R    8,192 4578248656   16384     86.31 1809296.292619  1809296.384835  tp_osd_tp    22191  R    8,192 4617494160   65536     92.22 1809295.165451  1809296.393715  bstore_kv_fi 17266  WS   8,192 1355703120   4096    1228.26 1809295.168595  1809296.401560  bstore_kv_fi 17266  WS   8,192 1122200      4096    1232.96 1809295.165221  1809296.408018  bstore_kv_fi 17266  WS   8,192 960656       4096    1242.80 1809295.166737  1809296.411505  bstore_kv_fi 17266  WS   8,192 57682504     4096    1244.77 1809296.292352  1809296.418123  tp_osd_tp    22191  R    8,192 4579459056   32768    125.77

I'm confused why write with O_SYNC must writeback on the backend storage device?  And when I used bcache for a time,

the latency increased a lot.(The SSD is not very busy), There's some best practices on configuration?





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