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

[Martin Wilck]

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

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