Perftune revoke write cache tuning abilities

[vladzcloudius]

Add a framework to deprecate parameters (ignore given values) and then use it to deprecate --write-back-cache parameter.

[vladzcloudius]

On Thu, Feb 24, 2022, 12:23 PM Pavel Emelyanov @.***> wrote:

@.**** commented on this pull request.

In scripts/perftune.py https://github.com/scylladb/seastar/pull/1011#discussion_r814102832:

•      ...
  

•    }
  

• :param prog_args: parsed program arguments object
• :return: filtered program arguments
• """

• deprecated_arguments: map from a corresponding field in progs_args ("dest") to an argument name

• deprecated_args_map = {

•    # "dest field" : "parameter name"
  

• }
• args_dict = vars(prog_args)
• for dopt in deprecated_args_map.keys():

•    if dopt in args_dict:
  

•        print("WARNING: Using {} is deprecated!".format(deprecated_args_map[dopt]), file=sys.stderr)
  

•        args_dict[dopt] = None
  

This is not "classical" deprecation. It should warn the user that the option is no longer in favor, but continue using it. And only after some time it should be removed (and ignored).

This patch'es motivation is not to depreciate this option - it's sent because I believe that this tweaking is generally dangerous and should not be used.

And today we saw a real-life proof of that. We saw how usage of this option on GCE caused await times of seconds level.

The moment it was disabled await times went to normal.

Lubos is going to file a separate GH issue with details.

We are going to stop any usage of this option on every Field Cloud GCE cluster effective immediately.

And on every non-GCE setup this option should never be used anyway.

It was a mistake to allow tweaking this parameter.

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[vladzcloudius]

@tarzanek FYI