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RAID PCI bus saturation question

TeeJayHoward

TeeJayHoward

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Will a 10 drive software RAID-5(or 6) setup saturate the PCI bus on my BFGRN4U? If so, what kind of a performance hit would it generate?

I'm looking at about $250 for a controller card or two with enough ports for all these drives, too... Right now the AOC-SAT2-MV8 seems to be the only option cheap enough. Does anyone have experience mix-and-match-ing controllers? Eg. Could I use two of my onboard ports and all eight on the PCI-X card?
 
Well look at it this way, pci maxes out at 133MB/s theoretical, lose about 10 percent for overhead, factor in other devices, like nic, sound card. You maybe have 50-80 MB/s left over. You could probably get bursts up to max rate pretty easily.

Now lets use a single, slower drive can probably do 40MB/s. You won't get that from all drives in the array, I'd say a 10 drive array with raid 5 or 6 could probably push 300MB/s if setup right on a good controller. Even if you get half that your pci bus will be completely saturated from the array alone.

It's going to really limit performance of the array. You also need to consider what you're using the array for though, and if that limitation will be an issue. If you're serving files over 100mbit lan, you only need a max rate from the array of about 12MB/s(max throughput of 100mbit).

You will not be able to spread the array across the card, and the onboard controllers. Some raid cards will allow to you span across multiple cards of the same type but not between other manufacturers or controller setups.
 
defakto said:
You will not be able to spread the array across the card, and the onboard controllers. Some raid cards will allow to you span across multiple cards of the same type but not between other manufacturers or controller setups.
Click to expand...
...Even with software RAID?
 
TeeJayHoward said:
...Even with software RAID?
Click to expand...

Software raid should work across cards, as it doesn't really matter how they are connected as long as the OS sees them.
 
The way I interpreted purchasing a card and software raid was an inexpensive card that does it's raid calculations through drivers, lacking an onboard hardware chip for that function.

If you're talking just putting all drives on the card, not setting up a raid through the cards bios, then setting up a software raid using windows dynamic disks, that's another story. Also an option I'd recommend against.
 
realize that with software RAID one will be even more limited by the PCI bus, especially for writes, since all the data needs to go across it and on writes it will pass through 2-3 times...
 
Software RAID is ok if you're only looking for data security, to a degree, but performance is typically really going to hurt. Unless you're putting that card into a true PCI-X slot or at least a 66MHz+ or 64bit slot you're gonna saturate the bus fairly fast. More so with software since it has to transfer things multiple times.

Also software options tend to take file A send part 1 to disk 1, part 2 to disk 2... and things would happen sequentially, so 4 seperate writes. So disk 1 would start writing, then disk 2, then disk 3 etc. With hardware and a higher level PCI-X setup you'd send the entire file at once in a single chunk, then start writing to all 4 disks at the very same time. The bus speed is going to be what makes a substantial difference in just how fast things go. Also keep in mind this is a rough simplification.

Also some boards have seperate PCI busses so you could saturate one off those while other components still work relatively unimpeded.
 
Why not get 2 1x controllers? Or less drives? Or maybe a better mobo?
 
TeeJayHoward said:
Does anyone have experience mix-and-match-ing controllers? Eg. Could I use two of my onboard ports and all eight on the PCI-X card?
Click to expand...

the only thing that I can come up with on the top of my head is that mix-and-match-ing controllers is likely to increase system complexity, thereby decreasing reliability.
 
aznx said:
Why not get 2 1x controllers? Or less drives? Or maybe a better mobo?
Click to expand...

1) I cannot locate PCIe 1x controllers with 8 ports for cheap.
2) Less drives = less storage space, or more cost. Can't quite afford it.
3) I COULD do this... But it would mean less money I can put towards drives/controllers, and I'd probably have to lose my overclock.
 
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