I am trying to benchmark I/O performance on my host and docker container using flexible IO tool with O_direct enabled in order to bypass memory caching. The result is very suspicious. docker performs almost 50 times better than my host machine which is impossible. It seems like docker is not bypassing the caching at all. even if I ran it with --privileged mode. This is the command I ran inside of a container, Any suggestions?
fio --name=seqread --rw=read --direct=1 --ioengine=libaio --bs=4k --numjobs=1 --size=10G --runtime=600 --group_reporting --output-format=json >/home/docker/docker_seqread_4k.json
The result is very suspicious. docker performs almost 50 times better than my host machine which is impossible. It seems like docker is not bypassing the caching at all.
If your best case latencies are suspiciously small compared to your worst case latencies it is highly likely your suspicions are well founded and that kernel caching is still happening. Asking for O_DIRECT is a hint not an order and the filesystem can choose to ignore it and use the cache anyway (see the part about "You're asking for direct I/O to a file in a filesystem but...").
If you have the option and you're interested in disk speed, it is better to do any such test outside of a container (with all the caveats that implies). Another option when you can't/don't want to disable caching is ensure that you do I/O that is at least two to three times the size (both in terms of amount and the region being used) of RAM so the majority of I/O can't be satisfied by buffers/cache (and if you're doing write I/O then do something like end_fsync=1 too).
In summary, the filesystem being used by docker may make it impossible to accurately do what you're requesting (measure the disk speed by bypassing cache while using whatever your default docker filesystem is).