[David Sterba <dave@jikos.cz>]

On Fri, Aug 17, 2012 at 09:45:20AM +0800, Liu Bo wrote:
> On 08/15/2012 06:12 PM, Lluís Batlle i Rossell wrote:
> > some time ago we discussed on #btrfs that the nocow attribute for files wasn't
> > working (around 3.3 or 3.4 kernels). That was evident by files fragmenting even
> > with the attribute set.
> > 
> > Chris mentioned to find a fix quickly for that, and posted some lines of change
> > into irc. But recently someone mentioned that 3.6-rc looks like still not
> > respecting nocow for files.
> > 
> > Is there really a fix upstream for that? Do nocow attribute on files work for
> > anyone already?
> > 
> 
> Dave had post a patch to fix it but only enabling NOCOW with zero sized file.
> 
> FYI, the patch is http://article.gmane.org/gmane.comp.file-systems.btrfs/17351
> 
> With the patch, you don't need to mount with nodatacow any more :)
> 
> And why it is only for only zero sized file:
> http://permalink.gmane.org/gmane.comp.file-systems.btrfs/18046

the original patch http://permalink.gmane.org/gmane.comp.file-systems.btrfs/18031
did two things, the reasoning why it is not allowed to set nodatasum in
general applies only to the second hunk but this

@@ -139,7 +139,7 @@ void btrfs_inherit_iflags(struct inode *inode, struct inode *dir)
 	}

 	if (flags & BTRFS_INODE_NODATACOW)
-		BTRFS_I(inode)->flags |= BTRFS_INODE_NODATACOW;
+		BTRFS_I(inode)->flags |= BTRFS_INODE_NODATACOW | BTRFS_INODE_NODATASUM;

 	btrfs_update_iflags(inode);
 }
---

is sufficient to create nocow files via a directory with NOCOW attribute
set, and all new files will inherit it (they are automatically
zero-sized so it's safe). This usecase is similar to setting the
COMPRESS attribute on a directory and all new files will inherit the
flag.

If Andrei wants to resend just this particular hunk, I'm giving it my ACK.


david