linux-kernel - Re: [PATCH 0/2] dm mpath: Interface for explicit probing of active paths

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Message-ID: <aCQiz88HksKg791Z@infradead.org>
Date: Tue, 13 May 2025 21:57:51 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Kevin Wolf <kwolf@...hat.com>
Cc: Christoph Hellwig <hch@...radead.org>, Martin Wilck <mwilck@...e.com>,
	dm-devel@...ts.linux.dev, hreitz@...hat.com, mpatocka@...hat.com,
	snitzer@...nel.org, bmarzins@...hat.com,
	linux-kernel@...r.kernel.org, pbonzini@...hat.com
Subject: Re: [PATCH 0/2] dm mpath: Interface for explicit probing of active
 paths

On Tue, May 13, 2025 at 11:29:09AM +0200, Kevin Wolf wrote:
> Am 13.05.2025 um 07:55 hat Christoph Hellwig geschrieben:
> > On Mon, May 12, 2025 at 05:18:43PM +0200, Kevin Wolf wrote:
> > > Yes, it's a bit unfortunate, but we have to work with what we have. QEMU
> > > doesn't even necessarily know that it's dealing with a multipath device,
> > > so it just has to blindly try the ioctl and see if it works.
> > 
> > Why is qemu even using SG_IO to start with?
> 
> How else would you do SCSI passthrough?
> 
> Ok, from your replies to Hannes I understand an implicit message, you
> wouldn't. But I don't think that's really an answer, at least not for
> all users.

SG_IO is fine and the only way for SCSI passthrough.  But doing
SCSI passthrough through md-multipath just doesn't work.  SCSI isn't
built for layering, and ALUA and it's vendor-specific variants and
alternatives certainly isn't.  If you try that you're playing with
fire and is not chance of ever moving properly.