Re: [dm-devel] RFC: one more time: SCSI device identification

From: Erwin van Londen <erwin@erwinvanlonden.net>
To: "Ewan D. Milne" <emilne@redhat.com>,
	Martin Wilck <martin.wilck@suse.com>,
	"Ulrich.Windl@rz.uni-regensburg.de"
	<Ulrich.Windl@rz.uni-regensburg.de>,
	"martin.petersen@oracle.com" <martin.petersen@oracle.com>
Cc: Hannes Reinecke <hare@suse.com>,
	"systemd-devel@lists.freedesktop.org"
	<systemd-devel@lists.freedesktop.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	"dgilbert@interlog.com" <dgilbert@interlog.com>,
	"jejb@linux.vnet.ibm.com" <jejb@linux.vnet.ibm.com>,
	"hch@lst.de" <hch@lst.de>
Subject: Re: [dm-devel] RFC: one more time: SCSI device identification
Date: Wed, 28 Apr 2021 10:09:25 +1000	[thread overview]
Message-ID: <455a6e5086831323af86a150d21d5a0a7c2299eb.camel@erwinvanlonden.net> (raw)
In-Reply-To: <c8ede601244e1710dbf320c33c0f7853e249bbee.camel@redhat.com>

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On Tue, 2021-04-27 at 16:41 -0400, Ewan D. Milne wrote:
> On Tue, 2021-04-27 at 20:33 +0000, Martin Wilck wrote:
> > On Tue, 2021-04-27 at 16:14 -0400, Ewan D. Milne wrote:
> > > 
> > > There's no way to do that, in principle.  Because there could be
> > > other I/Os in flight.  You might (somehow) avoid retrying an I/O
> > > that got a UA until you figured out if something changed, but
> > > other
> > > I/Os can already have been sent to the target, or issued before
> > > you
> > > get to look at the status.

If something happens on a storage side where a lun gets it's attributes
changed (any, doesn't matter which one) a UA should be sent. Also all
outstanding IO's on that lun should be returning an Abort as it can no
longer warrant the validity of any IO due to these changes. Especially
when parameters are involved like reservations (PR's) etc. If that does
not happen from an array side all bets are off as the only way to be
able to get back in business is to start from scratch.

> > 
> > 
> > Right. But in practice, a WWID change will hardly happen under full
> > IO
> > load. The storage side will probably have to block IO while this
> > happens, at least for a short time period. So blocking and
> > quiescing
> > the queue upon an UA might still work, most of the time. Even if we
> > were too late already, the sooner we stop the queue, the better.

I think in most cases when something happens on an array side you will
see IO's being aborted. That might be a good time to start doing TUR's
and if these come back OK do a new inquiry. From a host side there is
only so much you can do.

> > 
> > The current algorithm in multipath-tools needs to detect a path
> > going
> > down and being reinstated. The time interval during which a WWID
> > change
> > will go unnoticed is one or more path checker intervals, typically
> > on
> > the order of 5-30 seconds. If we could decrease this interval to a
> > sub-
> > second or even millisecond range by blocking the queue in the
> > kernel
> > quickly, we'd have made a big step forward.
> 
> Yes, and in many situations this may help.  But in the general case
> we can't protect against a storage array misconfiguration,
> where something like this can happen.  So I worry about people
> believing the host software will protect them against a mistake,
> when we can't really do that.

My thought exactly. 

> 
> All it takes is one I/O (a discard) to make a thorough mess of the
> LUN.
> 
> -Ewan
> 
> > 
> > Regards
> > Martin
> > 
> 
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