[Ocfs2-devel] [PATCH 1/3] fs: Document the reflink(2) system call.

[Joel Becker]

On Tue, May 05, 2009 at 02:19:07PM +0100, Jamie Lokier wrote:
> There was an attempt at something like that for ext3 a year or two ago.
> Search for "cowlink" if you're interested.

	Yeah, I discussed those with Jörn Engel after my talk at LSF - I
hadn't heard of them before.  cowlinks actually changed the semantic of
link(2).  This does not do that.

> Instead of a circular list, a proposed implementation was to create a
> separate "host" inode on the first reflink, converting the source
> inode to a reflink inode and moving the data block references to the
> new host inode.  Each reflink was simply a reference to the host
> inode, much like your design, and the host inode was only to hold the
> data blocks, with it's i_nlink counting the number of reflinks
> pointing to it.

	Reflinks are not cowlinks.  reflinks are new files (new inodes
in most implementations I expect) that only share the *data extents* in
a CoW fashion.
	Maybe reading the wiki details of the ocfs2 implementation and
so on would be helpful?

[Overview]
http://wiki.us.oracle.com/calpg/OCFS2Reflink
[ocfs2 Implementation]
http://oss.oracle.com/osswiki/OCFS2/DesignDocs/RefcountTrees
[reflink() Itself]
http://oss.oracle.com/osswiki/OCFS2/DesignDocs/ReflinkOperation
[Use Cases]
http://oss.oracle.com/osswiki/OCFS2/DesignDocs/ReflinkUses

Joel

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Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127