linux-kernel - RE: A blocksize problem about dax and ext4

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Message-ID: <94D0CD8314A33A4D9D801C0FE68B40295BEDCB5E@G9W0745.americas.hpqcorp.net>
Date:	Thu, 24 Dec 2015 12:26:18 +0000
From:	"Elliott, Robert (Persistent Memory)" <elliott@....com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Cholerae Hu <choleraehyq@...il.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Ted Tso <tytso@....edu>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: A blocksize problem about dax and ext4



> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@...radead.org]
> 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.");


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