Maths check - PCIe and SAS cards speed limits?

[Shrdlu]

Hi all,

I am trying to figure out which mother board to get for a build and want to check my maths for PCIe speeds with SAS cards.

The plan is to build an 8x8Tb ZFS-3 for Plex and storage with space for an additional 8 drives for future expansion and hot swaps. This would need 2 x LSI 9211-8i.

The board im looking at is Supermicro MBD-X11SSM-F which has 1 x PCIe 3.0 x8 and 2 x PCIe 3.0 x8 (x4 Bandwidth). But the LSI SAS HBA9211-8i cards are PCIe x8 running at x8. Obviously putting them into a x8 slot running at x4 will result in a speed drop.

But would this be a problem? x4 runs at 4GB/s while x8 runs at 8GB/s. With 8 x Ironwolfs each running at 250MB/s this only comes to 2GB/s so there should be no speed contraints? I only forsee a problem if running SSDs through this, which i wont be as they will be direct into the motherboard.

Does this make sense?

Thanks

 

[jgreco]

What makes you think that at any point in their daily operational life your HDD's will even approach 250MB/sec, much less all of them simultaneously?

The ONLY way that your HDD's get that speed is through long sequential operations at the outer edge. Once you start putting files onto a NAS, you start to get seeks, and your speeds drop into the much more typical 1MB/sec-50MB/sec range.

Really not a problem. ;-)

 

[Shrdlu]

Of course, 100% use is a very rare thing, but the limits are a nice thing to plan for. And not touching them is ideal haha! Thanks for the check. Just to ponder this for a while before buying then I will put up a thread of the build :)

 

[2nd-in-charge]

1 x PCIe 3.0 x8 and 2 x PCIe 3.0 x8 (x4 Bandwidth)

It also has 1xPCIe 3.0 x16 slot, I'm sure if you put one of the HBAs there it'll work at x8.

LSI SAS HBA9211-8i cards are PCIe x8 running at x8

They're also PCIe 2.0 so putting them in x4 slot will give you 4Gbit/s per lane, or 16Gbit/sec.
Still plenty for your movies over 1Gbit, or even 10Gbit network.

Also to avoid any PCIe bottlenecks and be ready for the time you'll put your movies on an array with 8 SSDs, consider getting 9207-8i, or anything else with SAS2308 chipset. It will run at PCIe 3.0 speed.

Also, if you're buying a new motherboard, you might be able to find one with a SAS2308 controller built in and a few SATA Ports.
This one, for example, although it's quite pricey. Or X11SSH-CTF-O, with SAS3008 on board, from the site you linked.