[dm-devel] [PATCH 04/19] Revert "multipath-tools: discard san_path_err_XXX feature"

[Martin Wilck]

On Wed, 2018-12-19 at 17:02 +0530, Muneendra Kumar M wrote:
> Hi Martin,
> In one of the patch   "[PATCH 00/19] san_path_err & multipath ANA
> support"
> 
> you have mentioned that san_path_err_XXX has some merits
> over marginal_path_err_XXX.
> 
> Is this understanding correct if so could you please explain the
> scenario
> in which use case this was better.
> 
> I can say Marginal_path_err_xx is superset of san_path_err_xx.

If you think so, please explain how. Imagine a user who has configured

  san_path_err_threshold     X
  san_path_err_forget_rate   Y
  san_path_err_recovery_time Z

Now this user is suppsed migrate to marginal_path settings.

  marginal_path_double_failed_time   A
  marginal_path_err_sample_time      B
  marginal_path_err_rate_threshold   C 
  marginal_path_err_recheck_gap_time D

Can you provide a formula to calculate A,B,C,D such that the system
behaves the same way (or "better") than previously with X, Y, Z?

I have pondered this for a while and concluded that I can't.

> If we need both san_path_err_xx , Marginal_path_err_xx then so many
> configurations will really confuse the customers.

True, the many different options are confusing. However, I don't think
it becomes much worse by offering both methods. Both methods aren't
easy to understand by themselves. Once users understand that these two
parameter sets are mutually exclusive, I think they can deal with that.

What we really need is easier set-up of either method (think of 2-3
sets of reasobable pre-set parameter values for different scenarios). 
I believe most admins are so intimidated by the complexity of the
parameters and their interaction that they give up and use
delay_xx_checks instead, or nothing at all.

Unfortunately this is all based on guessing; we at least have no data
if users are trying these parameters and if yes, what they are using.

Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)