Re: Data Deduplication with the help of an online filesystem check

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

On Thu, Jun 04, 2009 at 10:49:19AM +0200, Thomas Glanzmann wrote:
> Hello Chris,
> 
> > >  My question is now, how often can a block in btrfs be refferenced?
> 
> > The exact answer depends on if we are referencing it from a single
> > file or from multiple files.  But either way it is roughly 2^32.
> 
> could you please explain to me what underlying datastructure is used to
> monitor if the block is still referenced or already free? Is a counter
> used, bitmap (but that can't be if is 2^32) or some sort of list? I
> assume that a counter is used. If this is the case, I assume when a
> snapshot for example is deleted the reference counter of every block
> that was referenced in the snapshot will be decremented by one. Is this
> correct or am I missing something here?

It is a counter and a back reference.  With Yan Zheng's new format work,
the limit is not 2^64.

When a snapshot is deleted, the btree is walked to efficiently drop the
references on the blocks it referenced.

>From a dedup point of view, we'll want the dedup file to hold a
reference on the file extents.  The kernel ioctl side of things will
take care of that part.

-chris