Re: Redundancy check using "echo check > sync_action": error reporting?

[Peter Rabbitson <rabbit+list@rabbit.us>]

Theodore Tso wrote:
> On Fri, Mar 21, 2008 at 03:52:31PM +0100, Peter Rabbitson wrote:
>> I was actually specifically advocating that md must _not_ do anything on 
>> its own. Just provide the hooks to get information (what is the current 
>> stripe state) and update information (the described repair extension). The 
>> logic that you are describing can live only in an external app, it has no 
>> place in-kernel.
> 
> Why not?  If md doesn't do anything on its own, then when it detects a
> disagreement between the data and the two parity blocks, it has two
> choices (a) return possibly incorrect data to the application, or (b)
> return an I/O error and cause the application to blow up.
> 
> <snip>
> 
> So what's the harm in having an option where md does exactly what ECC
> memory does, which is when it can fix things up, to do so?  I bet most
> system administrators would turn it on in a heartbeat.
> 

With ECC memory you are checking for inconsistency on _every_single_read_ 
whereas the md scrubbing happens at best once a month if the admin turned the 
feature on. Moreover when md actually detects a mismatch the overwhelming 
chance is nobody needs this block at this moment, and might not need it for 
days to come. I think what is eluding this thread is the fact that md does not 
read _any_ redundant blocks unless it absolutely has to. And when it has to - 
you already have a missing chunk and can not apply ECC techniques either.


Of course it would be possible to instruct md to always read all data+parity 
chunks and make a comparison on every read. The performance would not be much 
to write home about though.

Peter