Re: [PATCHv3] nvme-mpath: delete disk after last connection

From: Sagi Grimberg <sagi@grimberg.me>
To: Hannes Reinecke <hare@suse.de>, Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Keith Busch <keith.busch@wdc.com>,
	linux-nvme@lists.infradead.org,
	Daniel Wagner <daniel.wagner@suse.de>
Subject: Re: [PATCHv3] nvme-mpath: delete disk after last connection
Date: Wed, 5 May 2021 13:40:29 -0700	[thread overview]
Message-ID: <8a274c79-6db8-a21b-e60e-4e73a9d139b5@grimberg.me> (raw)
In-Reply-To: <6a43fe69-e73a-07f8-f050-8116add83074@suse.de>

>>>>> As stated in the v3 review this is an incompatible change.  We'll need
>>>>> the queue_if_no_path attribute first, and default it to on to keep
>>>>> compatability.
>>>>>
>>>>
>>>> That is what I tried the last time, but the direction I got was to treat
>>>> both, NVMe-PCI and NVMe-oF identically:
>>>> (https://lore.kernel.org/linux-nvme/34e5c178-8bc4-68d3-8374-fbc1b451b6e8@grimberg.me/)
>>>
>>> Yes, I'm not sure I understand your comment Christoph. This addresses an
>>> issue with mdraid where hot unplug+replug does not restore the device to
>>> the raid group (pci and fabrics alike), where before multipath this used
>>> to work.
>>>
>>> queue_if_no_path is a dm-multipath feature so I'm not entirely clear
>>> what is the concern? mdraid on nvme (pci/fabrics) used to work a certain
>>> way, with the introduction of nvme-mpath the behavior was broken (as far
>>> as I understand from Hannes).
>>>
>>> My thinking is that if we want queue_if_no_path functionality in nvme
>>> mpath we should have it explicitly stated properly such that people
>>> that actually need it will use it and have mdraid function correctly
>>> again. Also, queue_if_no_path applies really if all the paths are
>>> gone in the sense they are completely removed, and doesn't apply
>>> to controller reset/reconnect.
>>>
>>> I agree we should probably have queue_if_no_path attribute on the
>>> mpath device, but it doesn't sound right to default it to true given
>>> that it breaks mdraid stacking on top of it..
>>
>> If you want "queue_if_no_path" behavior, can't you just set really high
>> reconnect_delay and ctrl_loss_tmo values? That prevents the path from
>> being deleted while it is unreachable, then restart IO on the existing
>> path once connection is re-established.
>>
> Precisely my thinking.
> We _could_ add a queue_if_no_path attribute, but we can also achieve the
> same behaviour by setting the ctrl_loss_tmo value to infinity.
> Provided we can change it on the fly, though; but it not that's easily
> fixed.
> 
> In fact, that's what we recommend to our customers to avoid the bug
> fixed by this patch.

You can change ctrl_loss_tmo on the fly. How does that address the
issue? the original issue is ctrl_loss_tmo expires for fabrics? or
pci unplug (which ctrl_loss_tmo does not apply to it)?

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