> > doing these writes now sounds like a
> > lot of work.  I'm guessing that needing two full-path
> > updates to achieve this means you're talking about a
> > much greater write penalty.
> 
> Not all that much.  Each full-path update is still
> only a single write request to the disk, since all
> the path blocks (again, possibly excepting the
> superblock) are batch-written together, thus mostly
> increasing only streaming bandwidth consumption.

Ok, that took some thinking about.  I'm pretty new to ZFS, so I've only just 
gotten my head around how CoW works, and I'm not used to thinking about files 
at this kind of level.  I'd not considered that path blocks would be 
batch-written close together, but of course that makes sense.

What I'd been thinking was that ordinarily files would get fragmented as they 
age, which would make these updates slower as blocks would be scattered over 
the disk, so a full-path update would take some time.  I'd forgotten that the 
whole point of doing this is to prevent fragmentation...

So a nice side effect of this approach is that if you use it, it makes itself 
more efficient :D
 
 
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