linux-kernel - Re: [PATCH v2 0/2] Adding per-controller timeout support to nvme

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Date:   Thu, 25 Apr 2019 07:45:31 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>
Cc:     Jens Axboe <axboe@...com>, James Smart <james.smart@...adcom.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Keith Busch <keith.busch@...el.com>,
        Maximilian Heyne <mheyne@...zon.de>,
        Amit Shah <aams@...zon.de>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 0/2] Adding per-controller timeout support to nvme

On Wed, 2019-04-24 at 13:58 -0700, Sagi Grimberg wrote:
> > It isn't that the media is slow; the max timeout is based on the SLA
> > for certain classes of "fabric" outages. Linux copes *really* badly
> > with I/O errors, and if we can make the timeout last long enough to
> > cover the switch restart worst case, then users are a lot happier.
> 
> Well, what is usually done to handle fabric outages is having multiple
> paths to the storage device, not sure if that is applicable for you or
> not...

Yeah, that turns out to be impractical in this case.

> What do you mean by "Linux copes *really* badly with I/O errors"? What
> can be done better?

There's not a lot that can be done here in the short term. If file
systems get errors on certain I/O, then graceful recovery would be
complicated to achieve.

Better for the I/O timeout to be set higher than the known worst case
time for successful completion.

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