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Date:   Thu, 1 Jul 2021 13:34:42 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Martin Wilck <mwilck@suse.com>
Cc:     Christoph Hellwig <hch@lst.de>, Mike Snitzer <snitzer@redhat.com>,
        Alasdair G Kergon <agk@redhat.com>,
        Bart Van Assche <Bart.VanAssche@sandisk.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org, dm-devel@redhat.com,
        Hannes Reinecke <hare@suse.de>,
        Daniel Wagner <dwagner@suse.de>, linux-block@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Benjamin Marzinski <bmarzins@redhat.com>, nkoenig@redhat.com,
        emilne@redhat.com
Subject: Re: [PATCH v5 3/3] dm mpath: add CONFIG_DM_MULTIPATH_SG_IO -
 failover for SG_IO
Message-ID: <20210701113442.GA10793@lst.de>
References: <20210628151558.2289-1-mwilck@suse.com> <20210628151558.2289-4-mwilck@suse.com> <20210701075629.GA25768@lst.de> <de1e3dcbd26a4c680b27b557ea5384ba40fc7575.camel@suse.com>
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On Thu, Jul 01, 2021 at 12:35:53PM +0200, Martin Wilck wrote:
> I respectfully disagree. Users of dm-multipath devices expect that IO
> succeeds as long as there's at least one healthy path. This is a
> fundamental property of multipath devices. Whether IO is sent
> "normally" or via SG_IO doesn't make a difference wrt this expectation.

If you have those (pretty reasonable) expections don't use SG_IO.
That is what the regular read/write path is for.  SG_IO gives you
raw access to the SCSI logic unit, and you get to keep the pieces
if anything goes wrong.