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

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Date:	Thu, 24 Dec 2015 02:47:07 +0000
From:	"Elliott, Robert (Persistent Memory)" <elliott@....com>
To:	Cholerae Hu <choleraehyq@...il.com>,
	Dan Williams <dan.j.williams@...el.com>
CC:	Dave Chinner <david@...morbit.com>, Ted Tso <tytso@....edu>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.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: Cholerae Hu [mailto:choleraehyq@...il.com]
> Sent: Wednesday, December 23, 2015 8:36 PM
> Subject: Re: A blocksize problem about dax and ext4
...
> xfs will silently disable dax when the fs block size is too small,
> i.e. your mmap() operations are backed by page cache in this case.
> Currently the only indication of whether a mapping is DAX backed or
> not is the presence of the VM_MIXEDMAP flag ("mm" in the VmFlags field
> of /proc/<pid>/smaps)
> 
> 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.

---
Robert Elliott, HPE Persistent Memory