PMFS and Linux 4.0 DAX

[Matthew Wilcox]

On Thu, Apr 23, 2015 at 11:50:19AM +0100, Jack Harvard wrote:
> The recently released Linux 4.0 ext4 filesystem can be enabled with
> DAX which bypasses the page caches, looks it's not byte-addressable,
> but still using a block device/ram driver for accessing the PM.
> 
> Wondering whether there is a list of differences between ext4 with DAX and PMFS?

PMFS is buggy and unsuitable for use in a production environment.
We looked at fixing it, and decided it was unfixable without a rewrite.
DAX is the approach we decided to pursue instead.

The fact that it sits on top of a block device isn't terribly relevant ...
user data is read/written through byte accesses.  Filesystem metadata
still goes through the block I/O path, but we don't think there's a big
win to be had by converting the filesystem to do byte accesses instead.