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From: Logan Gunthorpe <logang@deltatee.com>
To: Hannes Reinecke <hare@suse.de>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org
Cc: Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>,
	Max Gurtovoy <maxg@mellanox.com>,
	Stephen Bates <sbates@raithlin.com>
Subject: Re: [PATCH v6 00/16] nvmet: add target passthru commands support
Date: Fri, 26 Jul 2019 11:07:44 -0600	[thread overview]
Message-ID: <8fd8813f-f8e1-2139-13bf-b0635a03bc30@deltatee.com> (raw)
In-Reply-To: <1f202de3-1122-f4a3-debd-0d169f545047@suse.de>



On 2019-07-26 12:23 a.m., Hannes Reinecke wrote:
> How do you handle subsystem naming?
> If you enable the 'passthru' device, the (nvmet) subsystem (and its
> name) is already created. Yet the passthru device will have its own
> internal subsystem naming, so if you're not extra careful you'll end up
> with a nvmet subsystem which doesn't have any relationship with the
> passthru subsystem, making addressing etc ... tricky.
> Any thoughts about that?

Well I can't say I have a great understanding of how multipath works, but...

I don't think it necessarily makes sense for the target subsynqn and the
target's device nqn to be the same. It would be weird for a user to want
to use the same device and a passed through device (through a loop) as
part of the same subsystem. That being said, it's possible for the user
to use the subsysnqn from the passed through device for the name of the
subsys of the target. I tried this and it works except for the fact that
the device I'm passing through doesn't set id->cmic.

> Similarly: how do you propose to handle multipath devices?
> Any NVMe with several paths will be enabling NVMe multipathing
> automatically, presenting you with a single multipathed namespace.
> How will these devices be treated?

Well passthru works on the controller level not on the namespace level.
So it can't make use of the multipath handling on the target system.

The one case that I think makes sense to me, but I don't know how if we
can handle, is if the user had a couple multipath enabled controllers
with the same subsynqn and wanted to passthru all of them to another
system and use multipath on the host with both controllers. This would
require having multiple target subsystems with the same name which I
don't think will work too well.

> Will the multipathed namespace be used for passthru?

Nope.

Honestly, I think the answer is if someone wants to use multipathed
controllers they should use regular NVMe-of as it doesn't really mesh
well with the passthru approach.

Logan

  reply	other threads:[~2019-07-26 17:08 UTC|newest]

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-25 17:23 [PATCH v6 00/16] nvmet: add target passthru commands support Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 01/16] chardev: factor out cdev_lookup() helper Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 02/16] chardev: introduce cdev_get_by_path() Logan Gunthorpe
2019-07-25 17:40   ` Greg Kroah-Hartman
2019-07-25 17:53     ` Logan Gunthorpe
2019-07-25 17:58       ` Matthew Wilcox
2019-07-25 18:08         ` Logan Gunthorpe
2019-07-25 18:08       ` Greg Kroah-Hartman
2019-07-25 18:14         ` Logan Gunthorpe
2019-07-25 18:27           ` Greg Kroah-Hartman
2019-07-25 18:36             ` Logan Gunthorpe
2019-07-25 19:02               ` Sagi Grimberg
2019-07-25 19:34                 ` Greg Kroah-Hartman
2019-07-25 19:37                   ` Sagi Grimberg
2019-07-25 19:43                     ` Greg Kroah-Hartman
2019-07-25 19:45                       ` Sagi Grimberg
2019-07-25 19:43                     ` Sagi Grimberg
2019-07-25 19:41                   ` Logan Gunthorpe
2019-07-25 19:00             ` Matthew Wilcox
2019-07-25 19:05               ` Sagi Grimberg
2019-07-25 19:11                 ` Matthew Wilcox
2019-07-25 19:24                   ` Logan Gunthorpe
2019-07-25 19:26                     ` Matthew Wilcox
2019-07-25 19:31                       ` Logan Gunthorpe
2019-07-25 23:55                     ` Al Viro
2019-07-26  4:29                       ` Sagi Grimberg
2019-07-26  7:13                         ` Greg Kroah-Hartman
2019-07-26 15:46                         ` Logan Gunthorpe
2019-07-25 19:31                   ` Sagi Grimberg
2019-07-25 18:10       ` Greg Kroah-Hartman
2019-07-25 18:16         ` Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 03/16] chardev: export cdev_put() Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 04/16] nvme-core: introduce nvme_get_by_path() Logan Gunthorpe
2019-07-25 17:50   ` Matthew Wilcox
2019-07-25 17:54     ` Logan Gunthorpe
2019-07-25 19:58       ` Keith Busch
2019-07-25 20:12         ` Sagi Grimberg
2019-07-25 20:28         ` Logan Gunthorpe
2019-07-25 20:31           ` Keith Busch
2019-07-25 20:37             ` Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 05/16] nvme-core: export existing ctrl and ns interfaces Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 06/16] nvmet: add return value to nvmet_add_async_event() Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 07/16] nvmet: make nvmet_copy_ns_identifier() non-static Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 08/16] nvmet-passthru: update KConfig with config passthru option Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 09/16] nvmet-passthru: add passthru code to process commands Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 10/16] nvmet-passthru: add enable/disable helpers Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 11/16] nvmet-core: allow one host per passthru-ctrl Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 12/16] nvmet-core: don't check the data len for pt-ctrl Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 13/16] nvmet-configfs: introduce passthru configfs interface Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 14/16] block: don't check blk_rq_is_passthrough() in blk_do_io_stat() Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 15/16] block: call blk_account_io_start() in blk_execute_rq_nowait() Logan Gunthorpe
2019-07-25 17:23 ` [PATCH v6 16/16] nvmet-passthru: support block accounting Logan Gunthorpe
2019-07-26  6:23 ` [PATCH v6 00/16] nvmet: add target passthru commands support Hannes Reinecke
2019-07-26 17:07   ` Logan Gunthorpe [this message]
2019-07-26 22:21     ` Sagi Grimberg
2019-07-26 22:37       ` Logan Gunthorpe
2019-07-26 23:13         ` Sagi Grimberg
2019-07-27  0:09           ` Logan Gunthorpe
2019-07-27  0:50             ` Stephen  Bates
2019-07-29 16:15               ` Sagi Grimberg
2019-07-29 16:17                 ` Logan Gunthorpe

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