Re: Btrfs and data nocow per inode basis

["Ted Ts'o" <tytso@mit.edu>]

On Tue, Jun 12, 2012 at 04:44:23PM -0400, Chris Mason wrote:
> On Tue, Jun 12, 2012 at 01:15:27PM -0600, Ted Ts'o wrote:
> > It appears the NOCOW_FL flag is currently a no-op in the 3.2 kernel?
> 
> It's not a noop, but it is only setting the NODATACOW flag.  It needs to
> set the nodatasum flag as well, just like the mount -o nodatacow mount
> option does.
> 
> I'll fix this up on the kernel side, thanks Ted.

Here's the final patch to e2fsprogs that will be going into 1.42.4:

commit 5a23c93aeb65d61892a47f8f27bffad38f4759ea
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Tue Jun 12 17:09:39 2012 -0400

    lsattr, chattr: add support for btrfs's No_COW flag
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/lib/e2p/pf.c b/lib/e2p/pf.c
index f03193c..e2f8ce5 100644
--- a/lib/e2p/pf.c
+++ b/lib/e2p/pf.c
@@ -49,6 +49,7 @@ static struct flags_name flags_array[] = {
 	{ EXT2_TOPDIR_FL, "T", "Top_of_Directory_Hierarchies" },
 	{ EXT4_EXTENTS_FL, "e", "Extents" },
 	{ EXT4_HUGE_FILE_FL, "h", "Huge_file" },
+	{ FS_NOCOW_FL, "C", "No_COW" },
 	{ 0, NULL, NULL }
 };
 
diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
index f46a1a9..fb3f7cc 100644
--- a/lib/ext2fs/ext2_fs.h
+++ b/lib/ext2fs/ext2_fs.h
@@ -301,6 +301,7 @@ struct ext2_dx_countlimit {
 #define EXT4_EXTENTS_FL 		0x00080000 /* Inode uses extents */
 #define EXT4_EA_INODE_FL	        0x00200000 /* Inode used for large EA */
 /* EXT4_EOFBLOCKS_FL 0x00400000 was here */
+#define FS_NOCOW_FL			0x00800000 /* Do not cow file */
 #define EXT4_SNAPFILE_FL		0x01000000  /* Inode is a snapshot */
 #define EXT4_SNAPFILE_DELETED_FL	0x04000000  /* Snapshot is being deleted */
 #define EXT4_SNAPFILE_SHRUNK_FL		0x08000000  /* Snapshot shrink has completed */
diff --git a/misc/chattr.1.in b/misc/chattr.1.in
index 92f6d70..5a57d2c 100644
--- a/misc/chattr.1.in
+++ b/misc/chattr.1.in
@@ -64,6 +64,15 @@ this file compresses data before storing them on the disk.  Note: please
 make sure to read the bugs and limitations section at the end of this
 document.
 .PP
+A file with the 'C' attribute set will not be subject to copy-on-write
+updates.  This flag is only supported on file systems which perform
+copy-on-write.  (Note: For btrfs, the 'C' flag should be only
+set on new or empty files.  If it is set on a file which already has
+data blocks, it is undefined when the blocks assigned to the file will
+be fully stable.  If the 'C' flag is set on a directory, it will have no
+effect on the directory, but new files created in that directory will
+the No_COW attribute.)
+.PP
 When a directory with the `D' attribute set is modified,
 the changes are written synchronously on the disk; this is equivalent to
 the `dirsync' mount option applied to a subset of the files.
@@ -159,8 +168,7 @@ maintained by Theodore Ts'o <tytso@alum.mit.edu>.
 .SH BUGS AND LIMITATIONS
 The `c', 's',  and `u' attributes are not honored 
 by the ext2 and ext3 filesystems as implemented in the current mainline
-Linux kernels.    These attributes may be implemented
-in future versions of the ext2 and ext3 filesystems.
+Linux kernels.
 .PP
 The `j' option is only useful if the filesystem is mounted as ext3.
 .PP
diff --git a/misc/chattr.c b/misc/chattr.c
index 8a2d61f..141ea6e 100644
--- a/misc/chattr.c
+++ b/misc/chattr.c
@@ -107,6 +107,7 @@ static const struct flags_char flags_array[] = {
 	{ EXT2_UNRM_FL, 'u' },
 	{ EXT2_NOTAIL_FL, 't' },
 	{ EXT2_TOPDIR_FL, 'T' },
+	{ FS_NOCOW_FL, 'C' },
 	{ 0, 0 }
 };