feat(xo-server/Xapi): make built-in concurrency limits configurable

[julien-f]

See xoa-support#2075

Check list

Check if done, if not relevant leave unchecked.

• PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• if UI changes, a screenshot has been added to the PR
• documentation updated
• CHANGELOG.unreleased.md:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • unit tests (e.g. cron/parse.spec.js)
  • if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • at least manual testing

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer (and only one)
4. if necessary, update your PR, and re- add a reviewer

From the Four Agreements:

1. Be impeccable with your word.
2. Don't take anything personally.
3. Don't make assumptions.
4. Always do your best.

[olivierlambert]

Should we also update our doc or should we wait for a next release to do so?

[julien-f]

@olivierlambert These are advanced settings which should probably not be touched by users, unless told to do so by us after analyzing their situation.

They may changed or be removed in the future.

I don't think we should add them to our documentation, config entries are either documented in sample.config.toml if they are intended to be changed by users or directly in config.toml if they should probably be left alone.

[olivierlambert]

okay but we should document our values then (in the config file), to explain why we selected them at the first place.

[julien-f]

https://xen-orchestra.com/docs/concurrency.html#best-choice

[olivierlambert]

Maybe I'm missing the explanation for vmExportConcurrency = 2?

[julien-f]

See #4742

[olivierlambert]

Okay but my original question was about the "why we selected those values" in the first place. I have no memories whatsoever, is a "better" value could be found or we are sure that's the best one?

[julien-f]

I believe you chose these values, the optimal values probably depend on multiple factors like the XCP-ng/CH version, the SR used, etc.