On Wed, Mar 10, 2021 at 08:36:06AM -0500, Neal Gompa wrote:
On Wed, Mar 10, 2021 at 8:02 AM Matthew Wilcox
> On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote:
> > Forgive my ignorance, but is there a reason why this isn't wired up to
> > Btrfs at the same time? It seems weird to me that adding a feature
> btrfs doesn't support DAX. only ext2, ext4, XFS and FUSE have DAX support.
> If you think about it, btrfs and DAX are diametrically opposite things.
> DAX is about giving raw access to the hardware. btrfs is about offering
> extra value (RAID, checksums, ...), none of which can be done if the
> filesystem isn't in the read/write path.
> That's why there's no DAX support in btrfs. If you want DAX, you have
> to give up all the features you like in btrfs. So you may as well use
> a different filesystem.
So does that mean that DAX is incompatible with those filesystems when
layered on DM (e.g. through LVM)?
Yes. It might be possible to work through RAID-0 or read-only through
RAID-1, but I'm not sure anybody's bothered to do that work.
Also, based on what you're saying, that means that DAX'd
would not be able to use reflinks on XFS, right? That'd put it in
similar territory as swap files on Btrfs, I would think.
You can use DAX with reflinks because the CPU can do read-only mmaps.
On a write fault, we break the reflink, copy the data and put in a