Re: [Question] ext4/xfs: Default behavior changed after per-file DAX
From: "Darrick J. Wong" <djwong@kernel.org>
Date: 2021-10-26 15:48:40
Also in:
linux-ext4, linux-fsdevel
On Tue, Oct 26, 2021 at 10:12:17PM +0800, JeffleXu wrote:
Hi, Recently I'm working on supporting per-file DAX for virtiofs [1]. Vivek Goyal and I are interested [2] why the default behavior has changed since introduction of per-file DAX on ext4 and xfs [3][4]. That is, before the introduction of per-file DAX, when user doesn't specify '-o dax', DAX is disabled for all files. After supporting per-file DAX, when neither '-o dax' nor '-o dax=always|inode|never' is specified, it actually works in a '-o dax=inode' way if the underlying blkdev is DAX capable, i.e. depending on the persistent inode flag. That is, the default behavior has changed from user's perspective. We are not sure if this is intentional or not. Appreciate if anyone could offer some hint.
Yes, that was an intentional change to all three filesystems to make the steps we expose to sysadmins/users consistent and documented officially: https://lore.kernel.org/linux-fsdevel/20200429043328.411431-1-ira.weiny@intel.com/ (local) (This was the first step; ext* were converted as separate series around the same time.) --D
[1] https://lore.kernel.org/all/YW2Oj4FrIB8do3zX@redhat.com/T/ (local) [2] https://lore.kernel.org/all/YW2Oj4FrIB8do3zX@redhat.com/T/#mf067498887ca2023c64c8b8f6aec879557eb28f8 (local) [3] 9cb20f94afcd2964944f9468e38da736ee855b19 ("fs/ext4: Make DAX mount option a tri-state") [4] 02beb2686ff964884756c581d513e103542dcc6a ("fs/xfs: Make DAX mount option a tri-state") -- Thanks, Jeffle