linux-kernel - Re: [EXT] Re: [REGRESSION] qla2xxx: tape drive not removed after unplug FC cable

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Message-ID: <alpine.LRH.2.21.9999.2206221557150.4730@mvluser05.qlc.com>
Date:   Wed, 22 Jun 2022 16:03:49 -0700
From:   Arun Easi <aeasi@...vell.com>
To:     Tony Battersby <tonyb@...ernetics.com>
CC:     Thorsten Leemhuis <regressions@...mhuis.info>,
        Saurav Kashyap <skashyap@...vell.com>,
        Nilesh Javali <njavali@...vell.com>,
        <GR-QLogic-Storage-Upstream@...vell.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <regressions@...ts.linux.dev>
Subject: Re: [EXT] Re: [REGRESSION] qla2xxx: tape drive not removed after
 unplug FC cable

Hi Tony,

On Wed, 22 Jun 2022, 7:56am, Tony Battersby wrote:

> On 6/21/22 18:05, Arun Easi wrote:
> > Thanks for the info. Just to reiterate, you've reported two issues (though 
> > this log was showing only 1 of them).
> >
> > Issue 1 - Tape device never disappears when removed
> > Issue 2 - When a direct connected tape 1 was replaced with tape 2, tape 2 
> >           was not discovered.
> >
> > For Issue-2, please try the attached patch. This may not be the final fix, 
> > but wanted to check if that would fix the issue for you.
> >
> > For Issue-1, the behavior was intentional, though that behavior needs 
> > refinement. These tape drives support something called FC sequence level 
> > error recovery (added in FCP-2), which can make tape I/Os survive even 
> > across a short cable pull. This is not a simple retry of the I/O, rather a 
> > retry done at the FC sequence level that gives the IO a better chance of
> > revival. In other words, the said patch that caused regression, while 
> > introduces an incorrect reporting of the state of the device, makes backup 
> > more resilient.
> >
> > Now, onto the behavior when device state is reported immediately. What we 
> > have observed, at least with one tape drive from a major vendor, is that, 
> > across a device loss and device back case with both the events reported to 
> > upper layers, the backup operation was getting failed. This is due to a 
> > REPORT LUNS command being issued during device reappearance reporting 
> > (fc_remote_port_add -> SCSI scan), which the tape drive was not expecting 
> > and caused the backup to fail.
> >
> > I know that some tape drives do not support multiple commands to it at the 
> > same time, but not sure if that is still the norm these days.
> >
> > So, perhaps one way to make the behavior better, is to either report the 
> > disappearing device a bit delayed or have intelligence added in SCSI scan 
> > to detect ongoing tape IO operations and delay/avoid the REPORT LUNs. 
> > Former is a more contained (in the LLD) fix.
> >
> > Regards,
> > -Arun
> 
> Your patch does fix Issue-2 for me.  For Issue-1, it would be fine with
> me if qla2xxx reported device removal to the upper level a bit delayed,
> as you said.
> 

Thanks for testing and verifying the patch. We will post the patch 
upstream after due testing.

Regards,
-Arun