Re: Btrfs and data nocow per inode basis

[Chris Mason <chris.mason@fusionio.com>]

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.

-chris

> 
> <tytso.root@tytso-glaptop.cam.corp.google.com> {/mnt}  
> 2062# grep /mnt /proc/mounts 
> /dev/mapper/funarg-btrfs /mnt btrfs rw,relatime,space_cache 0 0
> <tytso.root@tytso-glaptop.cam.corp.google.com> {/mnt}  
> 2063# sync ; filefrag -v a
> Filesystem type is: 9123683e
> File size of a is 32768 (8 blocks, blocksize 4096)
>  ext logical physical expected length flags
>    0       0     3096               8 eof
> a: 1 extent found
> <tytso.root@tytso-glaptop.cam.corp.google.com> {/mnt}  
> 2064# dd if=/dev/zero of=a bs=32k conv=notrunc,nocreat count=1 
> 1+0 records in
> 1+0 records out
> 32768 bytes (33 kB) copied, 0.000119266 s, 275 MB/s
> <tytso.root@tytso-glaptop.cam.corp.google.com> {/mnt}  
> 2065# sync ; filefrag -v a
> Filesystem type is: 9123683e
> File size of a is 32768 (8 blocks, blocksize 4096)
>  ext logical physical expected length flags
>    0       0     3088               8 eof
> a: 1 extent found
> <tytso.root@tytso-glaptop.cam.corp.google.com> {/mnt}  
> 2066# lsattr a
> ---------------C a
> 
> I've attached the patch which I was going to commit, but when I tested
> it, it appears the flag is being set and displayed correctly, but
> btrfs doesn't appear to be honoring it.
> 
> Anyway want to explain what's going on?
> 
> 						- Ted
> 
> diff --git a/lib/e2p/pf.c b/lib/e2p/pf.c
> index f03193c..69181e7 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", "Huge_file" },
>  	{ 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..18b44bd 100644
> --- a/misc/chattr.1.in
> +++ b/misc/chattr.1.in
> @@ -64,6 +64,10 @@ 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, obviously.
> +.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.
> 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 }
>  };
>  
> --
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