2120S Stripe - abysmal performance

[Scott Long]

Don Bowman wrote:
> 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

I can't remember how the 2100 BIOS works, I'll have to take a look.
I thought that it was an attribute that was set when the array is
created.

Scott