Re: FreeBSD 5.3b7and poor ata performance

On Oct 25, 2004, at 5:39 PM, Brad Knowles wrote:
> At 3:25 PM -0600 2004-10-25, Scott Long wrote:
>>                                       But as was said, there is always
>>  a performance vs. reliability tradeoff.
>
> 	Well, more like "Pick two: performance, reliability, price"  ;)

That sounds familiar.  :-)

If you prefer...			...consider using:
----------------------------------------------
performance, reliability:	RAID-1 mirroring
performance, cost: 		RAID-0 striping
reliability, performance:	RAID-1 mirroring (+ hot spare, if possible)
reliability, cost:			RAID-5 (+ hot spare)
cost, reliability:			RAID-5
cost, performance:			RAID-0 striping

>> And when you are talking about RAID-10 with a bunch of disks, you 
>> will indeed start seeing bottlenecks in the bus.
>
> 	When you're talking about using a lot of disks, that's going to be 
> true for any disk subsystem that you're trying to get a lot of 
> performance out of.

That depends on your hardware, of course.  :-)

There's a Sun E450 with ten disks over 5 SCSI channels in the room next 
door: one UW channel native on the MB, and two U160 channels apiece 
from two dual-channel cards which come with each 8-drive-bay extender 
kit.  It's running Solaris and DiskSuite (ODS) now, but it would be 
interesting to put FreeBSD on it and see how that does, if I ever get 
the chance.

> 	The old rule was that if you had more than four disks per channel, 
> you were probably hitting saturation.  I don't know if that specific 
> rule-of-thumb is still valid, but I'd be surprised if disk controller 
> performance hasn't roughly kept up with disk performance over time.

That rule dates back to the early days of SCSI-2, where you could fit 
about four drives worth of aggregate throughput over a 40Mbs ultra-wide 
bus.  The idea behind it is still sound, although the numbers of drives 
you can fit obviously changes whether you talk about ATA-100 or 
SATA-150.

-- 
-Chuck