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Date: Thu, 28 Oct 2021 11:29:08 -0500 From: Eric Sandeen <sandeen@...deen.net> To: Vivek Goyal <vgoyal@...hat.com>, Dave Chinner <david@...morbit.com> Cc: "Darrick J. Wong" <djwong@...nel.org>, JeffleXu <jefflexu@...ux.alibaba.com>, Theodore Ts'o <tytso@....edu>, adilger.kernel@...ger.ca, ira.weiny@...el.com, linux-xfs@...r.kernel.org, "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>, linux-fsdevel@...r.kernel.org, dan.j.williams@...el.com, Christoph Hellwig <hch@....de>, Dave Chinner <dchinner@...hat.com> Subject: Re: [Question] ext4/xfs: Default behavior changed after per-file DAX On 10/27/21 8:14 AM, Vivek Goyal wrote: > On Wed, Oct 27, 2021 at 09:33:17AM +1100, Dave Chinner wrote: ... > Hi Dave, > > Thanks for all the explanaiton and background. It helps me a lot in > wrapping my head around the rationale for current design. > >> It's perfectly reasonable. If the hardware doesn't support DAX, then >> we just always behave as if dax=never is set. > > I tried mounting non-DAX block device with dax=always and it failed > saying DAX can't be used with reflink. > > [ 100.371978] XFS (vdb): DAX unsupported by block device. Turning off DAX. > [ 100.374185] XFS (vdb): DAX and reflink cannot be used together! > > So looks like first check tried to fallback to dax=never as device does > not support DAX. But later reflink check thought dax is enabled and > did not fallback to dax=never. We need to think hard about this stuff and audit it to be sure. But, I think that reflink check should probably just be removed, now that DAX files and reflinked files can co-exist on a filesystem - it's just that they can't both be active on the /same file/. I think that even "dax=always" is still just "advisory" - it means, try to enable dax on every file. It may still fail in the same ways as dax=inode (default) + flag set may fail. But ... we should go through the whole mount option / feature set / device capability logic to be sure this is all consistent. Thanks for pointing it out! -Eric >> IO
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