On Sep 10, 2009, at 2:44 PM, Majed B. wrote:
The maximum throughput you'll get is the PCI bus's speed. Make sure to note which version your server has. The silicon image controller will be your bottleneck here, but I don't have any numbers to say how much of a loss you'll be at. You'd have to search around for those who already benchmarked their systems, or buy/request a card to test it out.
I've actually been doing some of those benchmarks here. Given a Silicon Image 3124 card in a x1 PCI-e slot, my maximum throughput should be about 250MB/s (PCI-e limitation). My drives behind the pm are all capable of about 80MB/s, and I have 4 drives. What I've found is that when accessing one drive by itself, I get 80MB/s. When accessing more than one drive, I get a total of about 120MB/s, but it's divided by however many drives I'm accessing. So, two drives is roughly 60MB/s each, 3 drives about 40MB/s each, and 4 drives about 30MB/s each.
This is then complicated by whether or not you have motherboard ports in the same raid array. As the motherboard ports all get simultaneous drive speed more or less (up to 500MB/s aggregate in my test machine anyway), it's worth noting that the motherboard drives slow down to whatever speed you are getting on the drives behind the pm whenever they are combined. So, even if 5 drives on the motherboard could do 500MB/s total, 100MB/s each, if they are combined with 4 drives behind a pm at 30MB/s each, they switch down to 30MB/s each as well, and the combined total would then become 9 * 30MB/s for 270MB/s, considerably slower than just the 5 drives on the motherboard by themselves. However, if all your drives are behind pms, then I would expect to get a fairly linear speed increase as you increase the number of pms. You can then control how fast the overall array is by controlling how many drives are behind each pm up to the point that you reach PCI bus or memory or CPU bottlenecks.
If you do get a card and test it, make sure that you report back to us and update the wiki: http://linux-raid.osdl.org/index.php/Performance On Thu, Sep 10, 2009 at 9:35 PM, Drew<drew.kay@xxxxxxxxx> wrote:If you're looking at port multipliers, you need to find PCI-Express modules if you want them to be fast. The PCI ones are gonna be very slow when you have more than 2 disks per card.I'm definitely going to use the PCIX/PCIe slots for the Host Adapter. What I'm wondering is if I use a HBA and Port Multiplier that supportFIS based switching, say a Sil 3124 & 3726, how much of a loss in datatransfer rate can I expect from the RAID array built off the PM as opposed to each disk plugged in separately? An example configuration I'm looking at is a Sil3124 4 port HBA with three of the ports having Sil3726 5to1 PMs attached. Each PM then has four disks hung off the PM. If I create a RAID5 array for example oneach PM, what sort of speed degradation would I be looking at comparedto making a RAID5 off just the 3124? -- Drew "Nothing in life is to be feared. It is only to be understood." --Marie Curie-- Majed B. --To unsubscribe from this list: send the line "unsubscribe linux- raid" inthe body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
-- Doug Ledford <dledford@xxxxxxxxxx> GPG KeyID: CFBFF194 http://people.redhat.com/dledford InfiniBand Specific RPMS http://people.redhat.com/dledford/Infiniband
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