rbd cache did not help improve performance

[min fang <louisfang2013@xxxxxxxxx>]

Hi, I set the following parameters in ceph.conf

[client]
rbd cache=true
rbd cache size= 25769803776
rbd readahead disable after byte=0

map a rbd image to a rbd device then run fio testing on 4k read as the command
./fio -filename=/dev/rbd4 -direct=1 -iodepth 64 -thread -rw=read -ioengine=aio -bs=4K -size=500G -numjobs=32 -runtime=300 -group_reporting -name=mytest2

Compared the result with setting rbd cache=false and enable cache model, I did not see performance improved by librbd cache.

Is my setting not right, or it is true that ceph librbd cache will not have benefit on 4k seq read?

thanks.

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