Quick question on SAS2008 based RAID cards.

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brendanz

New Member
Jul 10, 2011
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I've got a question about the bandwidth usage on SAS2008 based controller cards.

I plan on building this on a Windows box, with RAID5, for a cheap, high capacity storage solution with a Norco 4224. I've recently purchased a 36 port HP SAS Expander from Odditory, and I'm looking to pair it up with a RAID5 HBA card.

I'm looking at the SAS2008 based card, either the LSI 9240-8i or the equivalent Intel RS2WC080. However, while it says it supports SAS drives @ 6Gbps, it says that it only supports SATA drives at 3Gbps. This is a little concerning for me, as I plan on using a bunch of cheap SATA3 drives (6Gbps).

When I am using the HP SAS Expander with an LSI 9240-8i, and SATA3 drives, does the expansion work out at either

a) 4x6Gbps = 24Gbps of bandwidth shared between the remaining 32 drive ports

OR

b) 4x3Gbps = 12Gbps of bandwidth shared between the remaining 32 drive ports

(In single link mode).

Thanks for your time!
 

nitrobass24

Moderator
Dec 26, 2010
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Yes it will not negotiate 6Gbps with each drive, but if you are using 7200RPM drives this is a non-issue.

As for total bandwith to the card from expander w/ single link you are looking at 12Gbps, Dual Link 24Gbps.
 

brendanz

New Member
Jul 10, 2011
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Thanks for quick response!

Hmmmm. I've got around 6 WD Caviar 2tb green drives, though in the future i plan on buying Deskstar 5400RPM 3tb drives.

Ok, another question for you if you don't mind! With the 12Gbps of bandwidth, I would guess that this would be enough for me if it was shared 100% equally amongst all the drives. However, does it actually work that way? Or does it work in a way of each individual expander port being allocated a certain amount of bandwidth?

For example, say I use a single link from the SAS2008 based card to the SAS expander, and i then plug in 3 SFF-8087 cables to 12 drives (4 drives each expansion port). If 4 of the drives on expansion port 1 are 100% utlised, do they share all 12Gbps of bandwidth equally between them? Or does expansion port 1 only get 4Gbps, because the other 8Gbps is allocated to expansion port 2 ans expansion port 3 (4Gbps each?)?

Thanks for your help :)
 

brendanz

New Member
Jul 10, 2011
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Bump :) if someone could help me out with this last bit of info I'd appreciate it as I am about to go through with my purchase of my new server :)

Thanks!! :)
 

RimBlock

Active Member
Sep 18, 2011
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All this Gbps is irritating and confusing :D.

I will convert to GB/MB per second for clarity across devices.

Ok, so to start, lets get the basics out, partly to answer your question but to give some background for others as well. Gbps -> MB/GB per sec conversions are not the same as for hard drive space where 8bits = 1 byte. Due to control and encoding of the data a Gbps -> GB/MB per sec is usually based on 10b encoding so you can divide by 10. The hard drive figures below are ball park to give an idea and not set in stone.

PCIe 1.0a = 250MB/s per channel (PCIe 1.0a 4x card will do 1GB/s max).
PCIe 2.0 = 500MB/s per channel (PCIe 2.0 4x card will do 2GB/s max).

SATA I (1.5Gbps) = 150MB/s
SATA II (300Gbps) = 300GB/s
SATA III (600Gbps) = 600GB/s
SAS (300Gbps) = 300GB/s
SAS2 (600Gbps) = 600GB/s

A 5.4k hard drive may give up to 70MB/s
A 7.2k hard drive may give up to 150MB/s
A SATA II SSD may give up to 250MB/s
A SATA III SSD may give up to 550MB/s

A SAS expander capable of SATA II on a single link to a controller will give 4x 300MB/s (1.2GB/s) max. Dual link will give 2.4GB/s. To follow through to the PCIe bus the controller will need to be able to handle the same max so for PCIe 1.0a on single link it will need to be a 5x card (so in reality a PCIe 8x) or for a PCIe 2.0 it needs to be a 3x (4x in reality). For dual link we are talking PCIe 1.0a needing 10x (so a 16x card) and PCIe 2.0 at 5x (so a 8x card).

Now on to the original question, the expander works like a network switch and so uses the bandwidth between the controller and expander up to the 300MB/s SATA II channel limit (4 channels per link) just as a network switch would use the 1Gbe connection to service all the traffic coming from the other ports destined for the machine on the other end but is limited to 1Gb/s per network cable (connection).

RB
 

Eiktyrner

New Member
Aug 2, 2011
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All this Gbps is irritating and confusing :D.
Yeah you have some strange numbers going on there... 300GB/s would be nice though ;)

SATA I (1.5 Gbps) = 150 MB/s
SATA II (3 Gbps) = 300 MB/s
SATA III (6 Gbps) = 600 MB/s
SAS (3 Gbps) = 300 MB/s
SAS2 (6 Gbps) = 600 MB/s