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[PATCH v2 1/6] dax: Add block size note to documentation

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  [PATCH v2 1/6] dax: Add block size note to documentation Matthew Wilcox <matthew.r.wilcox@intel.com> - 2015-07-03 16:50 +0200
    Re: [PATCH v2 1/6] dax: Add block size note to documentation Boaz Harrosh <openosd@gmail.com> - 2015-07-05 10:50 +0200

#1176632 — [PATCH v2 1/6] dax: Add block size note to documentation

From: Matthew Wilcox <willy@linux.intel.com>

For block devices which are small enough, mkfs will default to creating
a filesystem with block sizes smaller than page size.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
---
 Documentation/filesystems/dax.txt | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index baf4111..7af2851 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -18,8 +18,10 @@ Usage
 -----
 
 If you have a block device which supports DAX, you can make a filesystem
-on it as usual.  When mounting it, use the -o dax option manually
-or add 'dax' to the options in /etc/fstab.
+on it as usual.  The DAX code currently only supports files with a block
+size equal to your kernel's PAGE_SIZE, so you may need to specify a block
+size when creating the filesystem.  When mounting it, use the "-o dax"
+option on the command line or add 'dax' to the options in /etc/fstab.
 
 
 Implementation Tips for Block Driver Writers
-- 
2.1.4

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#1176881

On 07/04/2015 08:03 AM, Christoph Hellwig wrote:
> On Fri, Jul 03, 2015 at 10:40:38AM -0400, Matthew Wilcox wrote:
>> From: Matthew Wilcox <willy@linux.intel.com>
>>
>> For block devices which are small enough, mkfs will default to creating
>> a filesystem with block sizes smaller than page size.
> 
> This seems like an odd note.  At least for XFS and btrfs filesystem size
> don't change block sizes.  ff this is an extN oddity you're probably
> better off documenting it there.
> 

Is why I added to brd:
	/* This is so fdisk will align partitions on 4k, because of
	 * direct_access API needing 4k alignment, returning a PFN
	 * (This is only a problem on very small devices <= 4M,
	 *  otherwise fdisk will align on 1M. Regardless this call
	 *  is harmless)
	 */
	blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);

Perhaps add the same to pmem, and/or perhaps make mkfs.extX
also inspect this and not create blocks smaller then physical_block_size?

Thanks
Boaz

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