SAS confusion.

[dowxp]

Hi,
I'm planning a 12 drive raid 6 array using the following chassis:

http://www.supermicro.com/products/chassis/2U/826/SC826E2-R800LP.cfm

I'm confused on the following:

*I realize the backplane comes with a SAS expander already built in. but why does it advertise 28 port? It doesn't even have 28 bays .. I don't see the correlation.

*I'm looking at SAS raid controllers, for example, LSI describes their controllers as "X internal, and Y external". what does that mean?
http://www.lsi.com/channel/products/storagecomponents/Pages/6GBSATA_SASRAIDCards.aspx

*can a SFF8087 cable carry 4 SATA devices or somehow a number of SAS controllers? I don't see the connection.

I hope someone who knows more than me can clear this up. If someone can suggest the best option for a 12 drive raid 6 using SAS, I'd be really happy. 6G isn't required. 3G is good enough.

Thanks!

 

[mfenn]

*I realize the backplane comes with a SAS expander already built in. but why does it advertise 28 port? It doesn't even have 28 bays .. I don't see the correlation.

The SAS expander is basically a switch, so it needs upstream and downstream ports. Given that there are 12 drives and it takes a standard 4X SAS input, that accounts for 16 SAS ports. The reason that it is listed as 28 ports is that another common chassis form factor is 2U w/ 24 2.5" drives, so 24 + 4 = 28. The reason that there are two expander is that true SAS drives are dual-ported to allow for controller redundancy.

*I'm looking at SAS raid controllers, for example, LSI describes their controllers as "X internal, and Y external". what does that mean?
http://www.lsi.com/channel/products/storagecomponents/Pages/6GBSATA_SASRAIDCards.aspx

Internal means inside the case, external means that there is a port on the card's bracket for attaching external JBOD enclosures. Make sure that whatever controller you get has SFF8087 connectors on it (those are the most common) and supports SATA devices through SAS expanders. (quality controllers will).

*can a SFF8087 cable carry 4 SATA devices or somehow a number of SAS controllers? I don't see the connection.

A SFF8087 cable has 4 physical channels, which is why you commonly see breakout cables to four SATA. Your chassis has a backplane with expander, so you don't need to worry about that, just use an SFF8087 to SFF8087 cable to connect to the backplane.