diff mbox

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

Message ID 1435934443-17090-2-git-send-email-matthew.r.wilcox@intel.com
State New
Headers show

Commit Message

Wilcox, Matthew R July 3, 2015, 2:40 p.m. UTC
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(-)

Comments

Christoph Hellwig July 4, 2015, 5:03 a.m. UTC | #1
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.

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Boaz Harrosh July 5, 2015, 8:43 a.m. UTC | #2
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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diff mbox

Patch

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