linux-kernel - Re: [PATCH] nvme: make controller 'state' sysfs attribute pollable

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Message-ID: <daf5fe87-6b93-68aa-65be-b442615b9bd0@suse.de>
Date:   Mon, 25 Sep 2017 10:23:16 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Sagi Grimberg <sagi@...mberg.me>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Christoph Hellwig <hch@....de>
Cc:     Keith Busch <keith.busch@...el.com>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme: make controller 'state' sysfs attribute pollable

On 09/25/2017 09:09 AM, Sagi Grimberg wrote:
> 
> 
> On 25/09/17 08:59, Hannes Reinecke wrote:
>> On 09/25/2017 07:37 AM, Sagi Grimberg wrote:
>>>
>>>> So why exposing it then in the first time? I know you don't want
>>>> dm-mpath in
>>>> NVMe (neither do I) but we have to have something until your patchset
>>>> and ANA
>>>> is merged. And with this patch it's trivial to build a path checker
>>>> that just
>>>> looks at the state attribute in sysfs.
>>>
>>> Can't we just not use path-checkers for nvme (we already have one in
>>> nvme)?
>>
>> Really? For NVMe?
>> How would you do that, then?
> 
> Quick and dirty is to have a path-checker that returns path-up always,
> when the path go down, nvme will detect it and fast-fail the io.
> 
Well, yes; that's the trivial part.
But how do you know the path has become LIVE again?

>> Anyway: the entire point is that you don't _need_ a path checker for
>> NVMe.
>> The primary reason for path checkers is to check with the transport
>> layer if the remote endpoint is reachable.
>> (I know, that's not quite what they're doing now, but that's beside the
>> point).
>> For NVMf we do have KATO, so the NVMe subsystem knows exactly if the
>> connection is live or not. So it should be perfectly sufficient to check
>> the connection state instead of running a path checker of sorts.
>> But for doing so we need something in sysfs which we could check.
>>
>> Mind you, I wouldn't be adverse to have some common sysfs attribute,
>> with some common values (eg path up, path down, path blocked), and have
>> NVMf translating the internal state into that.
> 
> We could have such an interface I assume. But it would suck to maintain
> yet another state (we are already having enough trouble to have a
> coherent controller state machine).

Weell ... we could be using a notifier chain for that.
I think I should post my patchset.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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