linux-ext4 - Re: [PATCH 0/3] ext2, ext4, xfs: hard fail dax mount on unsupported devices

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Message-ID: <5a8e54e8-4845-1c85-e4e9-0b9b551a9ce2@sandeen.net>
Date:   Thu, 11 Oct 2018 13:38:34 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     Dan Williams <dan.j.williams@...el.com>, Jan Kara <jack@...e.cz>
Cc:     linux-xfs <linux-xfs@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 0/3] ext2, ext4, xfs: hard fail dax mount on unsupported
 devices



On 10/11/18 1:08 PM, Dan Williams wrote:
> On Thu, Oct 11, 2018 at 3:37 AM Jan Kara <jack@...e.cz> wrote:
>>
>> On Mon 08-10-18 14:32:46, Eric Sandeen wrote:
>>> In response to an earlier xfs patch to change how xfs reacts to
>>> dax incompatibilities, Dave said:
>>>
>>>> I suspect we need to be more harsh are rejecting mounts with -o dax
>>>> on devices DAX isn't supported on. This mount option is going into
>>>> production systems - it's not just for "testing" as the comments all
>>>> claim. i Things will break in production systems if DAX isn't
>>>> enabled and they are expecting it to be enabled.
>>>
>>> and I tend to agree, so proposing this change to hard-fail a dax mount if
>>> the device doesn't support it, instead of silently disabling the
>>> functionality.  Proposing for ext2, ext4, and xfs to keep behavior in
>>> sync.
>>
>> Let me include Dan and Ross into the discussion since they were the ones
>> proposing the "silent fallback" behavior (ext4 actually did fail the mount
>> instead not so long ago - see 24f3478d664b "ext4: auto disable dax instead
>> of failing mount" from December). Guys, why did you choose the fallback
>> path instead of a failure?
> 
> The different behavior between filesystems was confusing customers so
> we had to align them, then the question was which default to pick.
> Honestly, we came to the decision to bring ext4 in line with the xfs
> behavior because we thought that would be easier than the alternative.
> Dave and Christoph made repeated arguments that DAX is just a hidden
> performance optimization that no application should rely on, so we
> went the path of least resistance and changed the ext4 default.

Ok, well, I guess we'd better reconcile "it's a hidden performance hint"
with "if the administrator asked they must receive..." before making this
change... cc: hch for bonus input.

-Eric

> I'm perfectly fine switching both to the "fail if not DAX device" behavior.
>