configuration choices with Dell CERC (adaptec 2610SA)

[Scott Long]

user wrote:

> I am going to use a Dell CERC (adaptec 2610SA 6-port) (_not_ 21610sa
> 16-port) SATA raid card.
> 
> I am familiar with dell PERCs, and FreeBSD and RAID in generl, but I
> wanted to go over some of the configuration choices that the adaptec BIOS
> is giving me when I attempt to create a mirror:
> 
> Read caching: yes/no (default is yes)
> 
> (read caching does not rely on a battery, and is completely "safe",
> right?  I am going to set this to "yes", however I wonder - why would
> anyone _ever_ set it to "no" ?)
> 

The read cache really makes no difference, and sometime decreases
performance.  It forces the data to make two trips across the internal
adapter bus before it gets to host memory, and on many cards (i960-based
ones especially), this creates horrible bottlenecks.  Most modern OSes
have good read caching and read-ahead algorithms, and FreeBSD is no
exception.  In short, this option provides little benefit except in very
rare and specific situations.

Scott