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From: Sagi Grimberg <sagi@grimberg.me>
To: Christoph Hellwig <hch@lst.de>
Cc: Kanchan Joshi <joshi.k@samsung.com>,
	kbusch@kernel.org, axboe@kernel.dk, io-uring@vger.kernel.org,
	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	asml.silence@gmail.com, joshiiitr@gmail.com,
	anuj20.g@samsung.com, gost.dev@samsung.com
Subject: Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands
Date: Wed, 13 Jul 2022 14:00:56 +0300	[thread overview]
Message-ID: <772b461a-bc43-c229-906d-0e280091e17f@grimberg.me> (raw)
In-Reply-To: <20220713101235.GA27815@lst.de>


>> Maybe the solution is to just not expose a /dev/ng for the mpath device
>> node, but only for bottom namespaces. Then it would be completely
>> equivalent to scsi-generic devices.
>>
>> It just creates an unexpected mix of semantics of best-effort
>> multipathing with just path selection, but no requeue/failover...
> 
> Which is exactly the same semanics as SG_IO on the dm-mpath nodes.

I view uring passthru somewhat as a different thing than sending SG_IO
ioctls to dm-mpath. But it can be argued otherwise.

BTW, the only consumer of it that I'm aware of commented that he
expects dm-mpath to retry SG_IO when dm-mpath retry for SG_IO submission
was attempted (https://www.spinics.net/lists/dm-devel/msg46924.html).

 From Paolo:
"The problem is that userspace does not have a way to direct the command 
to a different path in the resubmission. It may not even have permission 
to issue DM_TABLE_STATUS, or to access the /dev nodes for the underlying 
paths, so without Martin's patches SG_IO on dm-mpath is basically 
unreliable by design."

I didn't manage to track down any followup after that email though...

>> If the user needs to do the retry, discover and understand namespace
>> paths, ANA states, controller states, etc. Why does the user need a
>> mpath chardev at all?
> 
> The user needs to do that for all kinds of other resons anyway,
> as we don't do any retries for passthrough at all.

I still think that there is a problem with the existing semantics for
passthru requests over mpath device nodes.

Again, I think it will actually be cleaner not to expose passthru
devices for mpath at all if we are not going to support retry/failover.

  reply	other threads:[~2022-07-13 11:01 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20220711110753epcas5p4169b9e288d15ca35740dbb66a6f6983a@epcas5p4.samsung.com>
2022-07-11 11:01 ` [PATCH for-next 0/4] nvme-multipathing for uring-passthrough Kanchan Joshi
     [not found]   ` <CGME20220711110800epcas5p3d338dd486fd778c5ba5bfe93a91ec8bd@epcas5p3.samsung.com>
2022-07-11 11:01     ` [PATCH for-next 1/4] io_uring, nvme: rename a function Kanchan Joshi
2022-07-14 13:55       ` Ming Lei
     [not found]   ` <CGME20220711110812epcas5p33aa90b23aa62fb11722aa8195754becf@epcas5p3.samsung.com>
2022-07-11 11:01     ` [PATCH for-next 2/4] nvme: compact nvme_uring_cmd_pdu struct Kanchan Joshi
2022-07-12  6:32       ` Christoph Hellwig
     [not found]   ` <CGME20220711110824epcas5p22c8e945cb8c3c3ac46c8c2b5ab55db9b@epcas5p2.samsung.com>
2022-07-11 11:01     ` [PATCH for-next 3/4] io_uring: grow a field in struct io_uring_cmd Kanchan Joshi
2022-07-11 17:00       ` Sagi Grimberg
2022-07-11 17:19         ` Jens Axboe
2022-07-11 17:18       ` Jens Axboe
2022-07-11 17:55         ` Sagi Grimberg
2022-07-11 18:22           ` Sagi Grimberg
2022-07-11 18:24             ` Jens Axboe
2022-07-11 18:58               ` Sagi Grimberg
2022-07-12 11:40               ` Kanchan Joshi
2022-07-14  3:40             ` Ming Lei
2022-07-14  8:19               ` Kanchan Joshi
2022-07-14 15:30                 ` Daniel Wagner
2022-07-15 11:07                   ` Kanchan Joshi
2022-07-18  9:03                     ` Daniel Wagner
     [not found]   ` <CGME20220711110827epcas5p3fd81f142f55ca3048abc38a9ef0d0089@epcas5p3.samsung.com>
2022-07-11 11:01     ` [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands Kanchan Joshi
2022-07-11 13:51       ` Sagi Grimberg
2022-07-11 15:12         ` Stefan Metzmacher
2022-07-11 16:58           ` Sagi Grimberg
2022-07-11 18:54           ` Kanchan Joshi
2022-07-11 18:37         ` Kanchan Joshi
2022-07-11 19:56           ` Sagi Grimberg
2022-07-12  4:23             ` Kanchan Joshi
2022-07-12 21:26               ` Sagi Grimberg
2022-07-13  5:37                 ` Kanchan Joshi
2022-07-13  9:03                   ` Sagi Grimberg
2022-07-13 11:28                     ` Kanchan Joshi
2022-07-13 12:17                       ` Sagi Grimberg
2022-07-14 15:14                   ` Ming Lei
2022-07-14 23:05                     ` Kanchan Joshi
2022-07-15  1:35                       ` Ming Lei
2022-07-15  1:46                         ` Ming Lei
2022-07-15  4:24                           ` Kanchan Joshi
2022-07-12  6:52       ` Christoph Hellwig
2022-07-12 11:33         ` Kanchan Joshi
2022-07-12 20:13         ` Sagi Grimberg
2022-07-13  5:36           ` Christoph Hellwig
2022-07-13  8:04             ` Sagi Grimberg
2022-07-13 10:12               ` Christoph Hellwig
2022-07-13 11:00                 ` Sagi Grimberg [this message]
2022-07-13 11:28                   ` Christoph Hellwig
2022-07-13 12:16                     ` Sagi Grimberg
2022-07-13 11:49                   ` Hannes Reinecke
2022-07-13 12:43                     ` Sagi Grimberg
2022-07-13 13:30                       ` Hannes Reinecke
2022-07-13 13:41                         ` Sagi Grimberg
2022-07-13 14:07                           ` Hannes Reinecke
2022-07-13 15:59                             ` Sagi Grimberg

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