Re: Problems with Promise PDC 20265 raid on ASUS Motherboard
On Sat, 06 Sep 2003, Roberto Sanchez wrote:
> > Don't use PDC20265 RAID. Instead, use software RAID1. It is much faster.
> > And safer, probably.
>
> How is that possible? The point of the hardware RAID is to hide the
For reading operations, the OS can do simultaneous reads of different data
from different disks (and Linux does exactly this). And your RAM is waaay
better a cache than what you have in your disks (and what you don't even
have in the PDC20265).
> implementation from the OS. The OS addresses the logical device and the
Since when hiding the implementation from the OS is something that should
make it faster? It is much *safer* if you are dealing with MS Windows crap,
but that's about it. It is also a very nice way to get vendor-locked to
your RAID controller.
> chipset takes care of the mirrorring/striping/whatever. For a software RAID,
If your RAID HW controller was a top-notch one, with lots of buffers and a
proper HW engine... maybe. But el-cheap-o Promise PDC20265 is worth very
little (I have an Asus A7V with a PDC20265. I talk from experience).
The RAID HW controllers I have used (Intel SCRU32 with 64MB cache and IBM
serverRAID with (I think) also 64MB cache) were not much faster when doing
raid 1 (these are SCSI disks, though, and tagged queues make a lot of
difference). They were better for RAID5, though. And these are high-end,
proper HW RAID controllers talking to SCSI-160 disks.
> IIRC, the OS has to legwork, meaning that for a mirror array it will send the
> daya to both disks. Maybe this is OK for a workstation, but I don't think it
> would cut it for very long on a server.
>
> I don't believe there is a major performance difference for striping,
> especially
> when you have only diks and they are different controllers, but I still think
> you are wrong for the most part.
If you have two disks, one in each channel of your PDC20265, running in
UDMA100, it *is* faster to use Linux software RAID in my experience (Asus
A7V, 256MB PC133 SDRAM, Athlon Thunderbird 1.2GHz).
If you doubt it, test it. Don't buy the "HW must be better than software"
crap, it is only true if you use the proper HW. And the PDC20265 ain't one
when compared to a fast processor and a proper OS with enough RAM.
Actually, it ain't a proper RAID HW controler at all in my book.
I have no idea whatsoever to what happens if you put the two disks in one
channel. I know software raid SUCKS in that configuration, but I seriously
doubt PDC20265 HW RAID would do any better. It does NOT support tag-queue
or disconnected operations, after all...
--
"One disk to rule them all, One disk to find them. One disk to bring
them all and in the darkness grind them. In the Land of Redmond
where the shadows lie." -- The Silicon Valley Tarot
Henrique Holschuh
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