On Wednesday, October 22, 2003, at 07:57  AM, Stephan Uphoff wrote:

> PS: Just for the record (feel free to ignore) as I don't think my
>     opinion is widely shared:
>     I don't like fs specific stuff in the block layer and would rather 
> see
>     a pseudo disk device for block interception being pushed on top of 
> the
>     disk device.

For snapshots, a shim device has at least two major problems:

	- Requires a priori knowledge that you will want to take
	  a snapshot at some point in the future (because you have
	  to insert the shim).

	- If you have an active snapshot, and then you reboot one day
	  and the shim is not inserted for some reason, then your
	  snapshot is now inconsistent and invalid.

...and a third reason, which isn't a show-stopper, but something I 
consider important nonetheless:

	- A single snapshot strategy may not be appropriate for all
	  file system types.  For example, a snapshot device would
	  be pretty much the perfectly WRONG approach for snapshotting
	  LFS.  I think snapshot code belongs in the file system,
	  because the file system can make better strategic decisions
	  than some generic shim that COW's blocks can.

         -- Jason R. Thorpe <thorpej@wasabisystems.com>