Re: Btrfs and data nocow per inode basis

[Goffredo Baroncelli <kreijack@inwind.it>]

On 06/12/2012 10:44 PM, 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.

Yes, if a filesystem is mounted with "nodatasum" options, the 'C' flags
works correctly.

So the solution it seems to set the BTRFS inode both as NODATASUM and
NODATACOW when chattr sets it as NOCOW.
Does make sense to allow to set NODATASUM alone ?


> 
> -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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