[dm-devel] [PATCH RFC] dm snapshot: shared exception store

[FUJITA Tomonori]

On Fri, 15 Aug 2008 01:43:55 -0700
Daniel Phillips <phillips at phunq.net> wrote:

> > A daemon program requests delta from the 
> > kernel, and sends it to another daemon program on the remote
> > server. The daemon on the remote server asks the kernel to apply
> > delta.
> 
> The downstream server just writes the delta to the origin, there is no
> need to ask the kernel to do this.
> 
> > The advantage of this approach, the above replication program can work
> > with any snapshot implementation, which could live in dm or file
> > systems like btrfs. File systems could implement the snapshot features
> > more efficiently than dm.
> 
> When you replicate a volume you can just send a list of changed blocks
> as ddsnap does.  This is not the case with a filesystem delta, which
> has to send the changed blocks of each filesystem object logically,
> along with relevant metadata such as changed permissions, ownership,
> file sizes etc.

What I think about is...

User-space replication programs don't know anything about delta. Delta
might be a list of changed blocks, or something more complicated. So
the downstream server can't simply write the delta to the origin. It
always asks the kernel.

User-space replication programs don't care about the format of
delta. A file system can give user-space programs whatever as
delta. All a file system needs to do is applying the delta that it
gave to user space.