[dm-devel] [PATCH 4/5] libmultipath: only read 0xc9 vpd page for devices with rdac checker

[Schremmer, Steven]

> -----Original Message-----
> From: Martin Wilck <martin.wilck at suse.com>
> Sent: Tuesday, November 3, 2020 3:00 AM
> To: bmarzins at redhat.com
> Cc: dm-devel at redhat.com; ng-eseries-upstream-maintainers <ng-eseries-upstream-maintainers at netapp.com>;
> christophe.varoqui at opensvc.com; Schremmer, Steven <Steve.Schremmer at netapp.com>
> Subject: Re: [PATCH 4/5] libmultipath: only read 0xc9 vpd page for devices with rdac checker
> 
> On Mon, 2020-11-02 at 19:11 -0600, Benjamin Marzinski wrote:
> > On Fri, Oct 30, 2020 at 09:12:46PM +0000, Martin Wilck wrote:
> > >
> > > Suggestion: we could try to retrieve VPD 0 (supported VPDs) before
> > > checking VPD 0xc9. That would avoid the errors on non-Netapp
> > > hardware,
> > > while still allowing us to autodetect RDAC systems that are missing
> > > in
> > > the hwtable.
> >
> > When this idea was brought up in discussions about how to solve this,
> > there was a worry that older rdac devices might not not support vpd
> > page
> > 0 correctly. I'm not sure how valid that worry is.
> 
> These old devices should be in the hwtable.
> 
> The algorithm would be something like this:
> 
>  A) if hwtable says "rdac", goto D)
>  B) if hwtable says nothing, goto E)
>  C) otherwise use hwtable settings, END
>  D) try VPD 0xc9
>     1) if it works as expected, use RDAC, END
>     2) otherwise, goto F)
>  E) try VPD 0
>     1) if it fails, goto F)
>     2) if it works and page c9 is listed, goto D)
>     3) if it works and c9 is not listed, goto F)
>  F) use defaults, END
> 
> I suppose "hwtable" means all built-in and user configuration here,
> including "overrides". But I haven't thought that through yet.
> 
> Cheers,
> Martin

I think Martin's algorithm would work, but it would also be safe to use
VPD 0 in all cases as rdac systems have supported page 0 for well over a 
decade. I think having "hwtable" include user configuration would be a
good choice, so that new rdac devices could be specified in multipath.conf.

Thanks,
Steve