Re: [PATCH 01/18] btrfs: create a mount option for dax
On Tue, Apr 16, 2019 at 9:42 AM Goldwyn Rodrigues <rgoldwyn(a)suse.de> wrote:
From: Goldwyn Rodrigues <rgoldwyn(a)suse.com>
This sets S_DAX in inode->i_flags, which can be used with
IS_DAX().
The dax option is restricted to non multi-device mounts.
dax interacts with the device directly instead of using bio, so
all bio-hooks which we use for multi-device cannot be performed
here. While regular read/writes could be manipulated with
RAID0/1, mmap() is still an issue.
Auto-setting free space tree, because dealing with free space
inode (specifically readpages) is a nightmare.
Auto-setting nodatasum because we don't get callback for writing
checksums after mmap()s.
Deny compression because it does not work with direct I/O.
I'd like to consider the dax mount option deprecated and require new
implementations to use a dynamic scheme that Darrick and I once
discussed where "dax" (or more specifically the MAP_SYNC semantic) is
an attribute that can be set on an empty directory and then inherited
by all files / directories created in that hierarchy.
Dax mappings are not always a performance win and enabling it for all
files in the filesystem is potentially overkill, and I'm of opinion
that we should step away from an filesystem-global mount option.