2120S Stripe - abysmal performance

[Don Bowman]

From: Scott Long [mailto:scottl at freebsd.org]
> Pete French wrote:
> 
> >>Read caching should be turned off unless you have a very 
> specific need
> >>for it.  Having it on is going to hurt sequential read 
> performance by a
> >>2x factor.  The only time is makes sense is when you have a 
> relatively
> >>small data set that gets read repeatedly, with few other 
> reads mixed in.
> > 
> > 
> > Interesting comment. Does that refer to this particular 
> controller, to
> > caches on RAID controllers in general, or to caches on the 
> drives themselves ?
> > 
> > -pete.
> 
> 
> With the read cache on, every time that the OS requests a new logical
> block that isn't in the cache, the controller has to first DMA that
> block from the disks into the cache, then DMA from the cache to host
> memory.  With the read cache off, it only has to DMA from the disks
> straight to host memory.  Some older AAC controllers also don't
> balance the cache well between read and write, so having both enabled
> winds up thrashing both.
> 
> The read cache on the drive is a good thing since it will do a bit of
> read-ahead which will help reduce latency.

Scott, how does one disable the read cache? Its not available
in the bios on the AAC (on the 2100). Is there a modepage for this?

--don