Re: Large latency with bcache for Ceph OSD

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:

quoted

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?