Re: Checksumming blocks? [was Re: the " 'official' point of view" expressed by kernelnewbies.org regarding reiser4 inclusion]

[Russell Leighton <russ@elegant-software.com>]

Thx, for a db this seems natural...

I am very curious about ZFS as I think we will seem more protection in 
the FS layer as disks get larger...
If I have a very old file I am now half way throught reading and ZFS 
finds a bad block, I assume I would
get some kind of read() error...but then what? Does anyone know if there 
are tools with ZFS to inspect the file?

Matthias Andree wrote:

> (I've stripped the Cc: list down to the bones.
> No need to shout side topics from the rooftops.)
>
> On Thu, 03 Aug 2006, Russell Leighton wrote:
>
>  
>
> > If the software (filesystem like ZFS or database like Berkeley DB)  
> > finds a mismatch for a checksum on a block read, then what?
> >    
>
> (Note that this assumes a Berkeley DB in transactional mode.) Complain,
> demand recovery, set the panic flag (refusing further transactions
> except close and open for recovery).
>
>  
>
> > Is there a recovery mechanism, or do you just be happy you know there is 
> > a problem (and go to backup)?
> >    
>
> Recoverability depends on log retention policy (set by the user or
> administrator) and how recently the block was written. There is a
> recovery mechanism.
>
> For applications that don't need their own recovery methods (few do),
> db_recover can do the job.
>
> In typical cases of power loss or kernel panic during write, the broken
> page will probably either be in the log so it can be restored (recover
> towards commit), or, if the commit hadn't completed but pages had been
> written due to cache conflicts, the database will be rolled back to the
> state before the interrupted transaction, effectively aborting the
> transaction.
>
> The details are in the Berkeley DB documentation, which please see.
>
>  

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