On 2/22/21 7:48 AM, Norman.Kern wrote: > 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.? > > Hmm, normally we won't observe the application issuing I/Os on backing device except for, - I/O bypass by SSD congestion - Sequential I/O request - Dirty buckets exceeds the cutoff threshold - Write through mode Do you set the write/read congestion threshold to 0 ? Coly Li > 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? >>
