Option to record alternate_epoch but keep zero-based timestamps

[aggieNick02]

Allow the option to set and record alternate_epoch but leave the
timestamps in all log files zero-based. This lets a user record the time
at which a job began with the desired clock_id while keeping nice
zero-based timestamps in the log files. (The zero-based timestamps are
easier to look at visually and also make the log files smaller, and
timestamps against the desired clock_id can be reconstructed by the user
if desired.)

The option log_alternate_eopch_clock_id is renamed to
alternate_epoch_clock_id. The new options relating to alternate_epoch
have not been in a released version of fio, so I expect this renaming
should be ok.

Signed-off-by: Nick Neumann nick@pcpartpicker.com

[vincentkfu]

I will take a closer look at this later but we typically try to maintain backward compatibility (unless there's a compelling reason otherwise) . Can you have the new preferred option name and the original one be synonyms? There is an alias field in struct fio_option that you can use for this.

[vincentkfu]

Also please add to the bottom of your commit message:

Fixes: #1544

[vincentkfu]

As far as I can see it should be possible to accomplish what you need by simply adding a new key in the JSON output that records when a job started. Then there is no need to change existing options. Would this work for you?

Consider an option called something like job_start_time_clock_id and the default could be either disabled or something else sensible. If this is enabled there would just be a new job start time key in the JSON output for each job.

[aggieNick02]

Thanks much for looking at this. I totally get not wanting to change option names - back when I wrote the PR the log_alternate_eopch_clock_id option was brand new and hadn't been in a tagged release of fio, but that's definitely not the case anymore.

What you described with job_start_time_clock_id sounds great and solves the problem well. It leaves the existing log_alternate_epoch_clock_id alone. I'll look at if it makes most sense for the default job_start_time_clock_id option to either not log anything, or use a sensible clock_id. I'll try to get the PR together quickly.

Very much appreciate the time and the feedback and idea. Thanks again!

[aggieNick02]

Suggested modified approach is up: #1617