FreeBSD 5.3b7and poor ata performance
Charles Swiger
cswiger at mac.com
Mon Oct 25 15:08:27 PDT 2004
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
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