Re: [PATCH 1/3] fs: Document the reflink(2) system call.

[Tao Ma <tao.ma@oracle.com>]

Hi Ted,

Theodore Tso wrote:
> On Sat, May 02, 2009 at 11:15:01PM -0700, Joel Becker wrote:
>> int reflink(const char *oldpath, const char *newpath);
>>
>> The reflink(2) system call creates reference-counted links.  It creates
>> a new file that shares the data extents of the source file in a
>> copy-on-write fashion.  Its calling semantics are identical to link(2).
>> Once complete, programs see the new file as a completely separate entry.
> 
> How should quota handle reflinks?  Since there are separate inodes,
> the two files could be owned by different user ID's.  Since the data
> blocks exist only once, I can imagine a number of different ways of
> handling it:
> 
> 1) When the reflink is created, the owner of the new reflink is not
> charged the number of blocks of the file against his/her quota.  If
> the original inode is deleted, the original owner continues to have
> the cost of the file charged against his/her quota until the last
> reflink disappears.
> 
> 2) When the reflink is created, the owner of the new reflink is NOT
> charged the number of blocks of the file against his/her quota.  If
> the original inode is deleted, the owner of the reflink is charged the
> number of blocks against his/her quota.  If that drives the owner over
> quota, the quota subsystem will enforce the soft and hard quota limits
> as per normal.  If there are more than one reflink against the file,
> the system will randomly choose one user and charge the blocks against
> his/her quota.
> 
> 3) When the reflink is created, the owner of the new reflink is
> charged the number of blocks of the file against his/her quota.  The
> original owner of the inode continus to also have the blocks of the
> file charged against his/her quota, so in effect the blocks are
> "double counted".
> 
> 4) When the reflink is created, the owner of the new reflink is NOT
> charged the number of blocks of the file against his/her quota.  The
> original owner of the inode continues to also have the blocks of the
> file charged against his/her quota; if the file is deleted the blocks
> associated with the file will not be charged against any users' quota.
> 
> All of these have various problems; and maybe the answer is that
> reflinks aren't really compatible with quotas, so pick something least
> bad (say #3), and we can just move on.
yeah, agree. So I will pick #3 in my ocfs2 reflink implementation.

Thanks.

Regards,
Tao