[Libguestfs] libnbd flamegraph Odd benchmark results

[Abhay Raj Singh]

> The question (for you!) is how we can choose better settings, and
> whether we should choose


 In my opinion configuration resolution will be best if resolved in the
following order of increasing priority/weightage.
1. Auto-detect depending on the system parameters.
Auto-detection of best params is a must IMO as there's
enough diversity in server hardware that one size won't fit
all can be implemented without much effort(excluding benchmarking).

2. Configuration file: A yaml, toml, or any other simple format
- Load a different config file with *nbdcopy --file=custom-config.yaml*
- Maybe generate the default config file having values from the auto-detect
phase.
nbdcopy --generate-config

~/.config/nbdcopy/config.yaml (used to get values)
~/.config/nbdcopy/example.yaml (generated for reference from auto)

3. CLI params
A quick way to override configuration file params
ex: functionally same
`nbdcopy --requests=4 --config=custom-config.yaml`
`nbdcopy --config=custom-config.yaml --requests=4`

The flow of initialization:
Read config file ----override ---> CLI params --- missing params -->
auto-detect fill
or
easier implementation:
auto-detect fill --- override ---> config file --- override ---> CLI params


The rationale for choosing configuration file method:
- Easier to use avoiding long commands (less typing, scripts easier to read)
- Easy switch between multiple configurations and override them.
- Documentation and configuration in the same place
So easier for someone else to understand why certain parameters were chosen.
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