Re: [PATCH] scsi_dh_alua: handle RTPG sense code correctly during state transitions

["Ewan D. Milne" <emilne@redhat.com>]

On Tue, 2019-10-08 at 08:21 +0200, Hannes Reinecke wrote:
> On 10/7/19 10:45 PM, Ewan D. Milne wrote:
> > 
> > The patch itself looks OK, but I was wondering about a couple of things:
> > 
> >   - There are other places in scsi_dh_alua where the ASC/ASCQ 04 0A is checked
> >     and we retry, I understand that this is a particular case you are solving
> >     but is the changing of the state to -> transitioning (because that's what
> >     the device said the state was) applicable in those other cases?
> 
> No. The original code was built around the assumption that RTPG would
> return the status of the device; consequently we would have to retry
> RTPG until we get a final status. But as mentioned, there are arrays
> which cannot return RTPG data during transitioning, so the code would
> never be able to detect a transitioning state.
> With this patch we set the state directly once the said sense code is
> received.
> But this applies _only_ to the RTPG command, as this is required to move
> the state machine along.
> None of the other commands are affected.
> 
> >   - The code originally seems to have been under the assumption that the
> >     transitioning state was a transient event, so the retry would pick up
> >     the eventual state.  Now, some storage arrays spend a long time in the
> >     transitioning state, but if we don't send another command are we going to
> >     get the sense (or the UA) that triggers entry to the eventual ALUA state?
> > 
> 
> Note, there are two types of retries.
> The one is the 'normal' command retry, where we resend a command a given
> number of times to retrieve the final status.
> This is precisely the error which caused this patch.
> 
> And then there is a scheduled retry; here we essentially poll the array
> with sending RTPG in regular intervals until the 'transitioning' state
> is gone. (Check for 'alua_rtpg()' and the handling of the SCSI_DH_RETRY
> return value). With the patch we continue to trigger that second type of
> retries, which will eventually clear the transitioning state.
> 
> Cheers,
> 
> Hannes

Thanks for the explanation.  The patch looks good.

Reviewed-by: Ewan D. Milne <emilne@redhat.com>