Problem about very high Average Read/Write Request Time

[quanjun hu]

Hi,
   I am using xfs on a raid 5 (~100TB) and put log on external ssd device,
the mount information is:
/dev/sdc on /data/fhgfs/fhgfs_storage type xfs
(rw,relatime,attr2,delaylog,logdev=/dev/sdb1,sunit=512,swidth=15872,noquota).
  when doing only reading / only writing , the speed is very fast(~1.5G),
but when do both the speed is very slow (100M), and high r_await(160) and
w_await(200000).
   1. how can I reduce average request time?
   2. can I use ssd as write/read cache for xfs?

Best regards,
Quanjun
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