Re: end to end error recovery musings

Ric Wheeler
2007-02-27T18:51:39Z

Martin K. Petersen wrote:
>>>>>> "Eric" == Moore, Eric <Eric.Moore@lsi.com> writes:
>
> Eric> Martin K. Petersen on Data Intergrity Feature, which is also
> Eric> called EEDP(End to End Data Protection), which he presented some
> Eric> ideas/suggestions of adding an API in linux for this.
>
> T10 DIF is interesting for a few things:
>
> - Ensuring that the data integrity is preserved when writing a buffer
> to disk
>
> - Ensuring that the write ends up on the right hardware sector
>
> These features make the most sense in terms of WRITE. Disks already
> have plenty of CRC on the data so if a READ fails on a regular drive
> we already know about it.

There are paths through a read that could still benefit from the extra
data integrity. The CRC gets validated on the physical sector, but we
don't have the same level of strict data checking once it is read into
the disk's write cache or being transferred out of cache on the way to
the transport...

>
> We can, however, leverage DIF with my proposal to expose the
> protection data to host memory. This will allow us to verify the data
> integrity information before passing it to the filesystem or
> application. We can say "this is really the information the disk
> sent. It hasn't been mangled along the way".
>
> And by using the APP tag we can mark a sector as - say - metadata or
> data to ease putting the recovery puzzle back together.
>
> It would be great if the app tag was more than 16 bits. Ted mentioned
> that ideally he'd like to store the inode number in the app tag. But
> as it stands there isn't room.
>
> In any case this is all slightly orthogonal to Ric's original post
> about finding the right persistence heuristics in the error handling
> path...
>

Still all a very relevant discussion - I agree that we could really use
more than just 16 bits...

ric