Re: [9fans] Venti and the hash / public key in plan9

[Ronald G Minnich <rminnich@lanl.gov>]

Lluís Batlle wrote:
> 2006/2/6, Russ Cox <rsc@swtch.com>:
> 
>>>  Anyway I've don't
>>>understood what does Venti do when a hash is found in the stored
>>>blocks, and the contents of the blocks are different (the
>>>low-probability case).  I imagine there is some code which does not
>>>give a data-loss...  Can someone give a small explanation about that?
>>
>>It responds to the write RPC with an error.
> 
> So what should manage that error? The application? Maybe it try a
> different write size?
> In fact there should be another application layer between 9p and the
> RPC for venti, isn't it? (which?)

Lluis, in a system like this, if two blocks have the same hash, they are 
by definition the same. It therefore makes no sense to say 'what if the 
data is different'. The data is not, from the viewpoint of venti. So a 
write of a block with hash X, when a block of hash X already exists in 
venti, is an error. It's the same data. So you get an error on the write 
RPC.

So your real question is, 'what if a block B' has the same hash as a 
block B? What does venti do?" It returns B, not B', as it should. That's 
the underlying rule. That's one reason you want a long hash, with a good 
design :-)

Now, let's see if I got this right :-)

ron