Re: [PATCH v1 5/7] xfs: Add device retry

[Allison Henderson]

On 11/28/18 5:41 AM, Bob Liu wrote:
> On 11/28/18 3:35 PM, Christoph Hellwig wrote:
> > On Wed, Nov 28, 2018 at 04:08:50PM +1100, Dave Chinner wrote:
> > > So the first time through this loop the block layer devices what
> > > device to read from, then we iterate devices 1..n on error.
> > >
> > > Whihc means if device 0 is the only one with good information in it,
> > > we may not ever actually read from it.
> > >
> > > I'd suggest that a hint of "-1" (or equivalent max value) should be
> > > used for "device selects mirror leg" rather than 0, so we can
> > > actually read from the first device on command.
> >
> > Yes.  For one thing I think we really need to split this retry counter
> > of sorts from the write hints.  I.e. make both u8 types and keep them
> > separate.  Then start out with (u8)-1 as initialized by the block layer
> > for the first attempt.  The device then fills out which leg it used
> > (in the completion path, so that another underlying driver doesn't
> > override it!), and then the file system just preserves this value on
> > a resumit, leaving the driver to chose a new value when it gets a
> > non -1 value.
>
> Will update as suggested, thank you for all your feedback :)
>
> -Bob

Yes, thanks everyone for your feed back.  Maybe Bob and I can come up 
with some test cases that recreate the problem scenarios described here 
and see if we can work out a solution to the multi bio complexities. 
Thanks!

Allison