RE: A blocksize problem about dax and ext4

["Elliott, Robert (Persistent Memory)" <elliott@xxxxxxx>]

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@xxxxxxxxxxxxx]
> Sent: Thursday, December 24, 2015 4:11 AM
> Subject: Re: A blocksize problem about dax and ext4
> 
> On Thu, Dec 24, 2015 at 02:47:07AM +0000, Elliott, Robert (Persistent
> Memory) wrote:
> > > Did you mean that I should make the blocksize bigger until the mount
> > > command tell me that dax is enabled?
> >
> > To really use DAX, the filesystem block size must match the
> > system CPU's page size, which is probably 4096 bytes.
> 
> No, it doesn't.  File you use for DAX must be aligne at page size
> granularity.  For XFS you could do this with the per-inode extent
> size hint for example even if the overall block size is smaller.

I think that's a future goal.

Currently, the checks are like this:

        if (sb->s_blocksize != PAGE_SIZE) {
                xfs_alert(mp,
                "Filesystem block size invalid for DAX Turning DAX off.");