Re: libata bridge limits

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Aug 26 2008, Alan Cox wrote:
> > a) Why was this limit put in there? It limits both transfer speed and
> >    request size. If it's due to some dodgy drive/bridge, perhaps we
> >    should just check for that and only apply the transfer limits when
> >    detected (or blacklisted). On the bridge setups I've seen, I've never
> >    had problems with killing the limit.
> 
> Various old bridges need it - and you can't detect the bridge type.

Not generically, but for some devices (like the Mtron) we can.

> > b) Put in a whitelist, easy to do for these Mtron drives.
> > 
> > c) Add a parameter to turn it on (or off, depending on the default) for
> >    a specific drive.
> > 
> > I'm in favor of a) personally, but I'd like to hear why the check was
> > added originally first. Dropping 20-30% of the throughput performance on
> > the floor without option seems like a really bad choice.
> 
> Can I suggest 
> 
> d) Assume the bridge is ok but teach the SATA error handling code that if
> there is a timeout immediately with such a bridge then to flip down to
> UDMA5 and knobble the transfer length.

That would be nice, assuming that we can rely on safe behaviour (eg not
data corrupting badness).

-- 
Jens Axboe