dd with oflag=direct,sync runs slower than dd with oflag=direct on a virtual disk with writeback cache enabled
Issue
- When testing disk i/o using dd, and using only "direct" in oflag, the "%util" in sar (or iostat, or other similar tools) is 10-20%
If "direct" and "sync" are used together in oflagh, "%util" reaches up to 90~100%.
Environment
- Observed in Red Hat Enterprise Linux 8.4 running as a guest OS in a qemu-kvm based VM.
- The difference in system behavior is not version specific, therefore other Red Hat Enterprise Linux versions are expected to be behaving in a similar way.
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