Re: FreeBSD 5.3b7and poor ata performance

On Oct 25, 2004, at 2:29 PM, Scott Long wrote:
>> Also, there is an unresolvable question. Why two 52MB/s disks
>> in raid0 has a throughput of 40MB/s and for raid1 18MB/s??
>
> Would you _PLEASE_ stop trying to associate RAID with performance!
> RAID is about reliability and reduncdancy, not about speed.

All RAID modes make tradeoffs between performance, reliability, and 
cost.

RAID-1 mirroring and RAID-5 provide higher reliability by using partial 
or full redundancy.  However, RAID-0 striping provides no additional 
reliability: the primary reason for using RAID-0 is to improve 
performance by accessing two or more devices in parallel.

> Some cases can give you desirable performance increases as a side 
> effect,
> but that is not the primary goal.

Disagree.  Why else would you use RAID-0 striping?

[ If you simply want to create a logical volume bigger than the size of 
a physical drive, you can use concatenation instead. ]

> Specifically in this case, the
> GEOM raid classes are fairly new and have not had the benefit of
> years of testing.  I'd much rather that the focus be on stability
> and reliability for them, not speed.  Once the primary goals of
> RAID are satisfied then we can start looking at performance.

Your position is certainly reasonable: if a storage system is not 
reliable, how fast it performs is something of a moot point.  :-)  
However, this being said, a RAID-0 implementation needs to improve 
performance compared with using a bare drive if it is to be useful.

-- 
-Chuck