educate me about SAS controller

[theinv]

Can someone educate me about SAS controllers? There are many things I don't understand about SAS controllers and I can't find good answers from the web.

Here is one example,

Areca ARC-1680ix-8 SAS RAID Controller

http://www.pc-pitstop.com/sas_controllers/arc1680ix8.asp

PCIe x8, Intel IOP348 1200MHz processor, 512mb cache,
Supports up to 128 devices using SAS expanders,
2xSFF-8087 internal connectors,
1xSFF-8088 external connector,
Low Profile, Retail Box with internal cables.
$715.00
Part#:SC-ARC1680ix-8

* Intel 1200 MHz IOP348 I/O processor with SAS controller
* 512mb cache memory
* Supports write-through or write-back cache
* Supports 8 internal and 4 external SAS ports

Disk Bus Interface

* Up to (128) SAS or 3 SATA II drives using SAS expanders
* Up to 3Gb/s per port

My questions:

1. connectors and ports

This Areca controller has 2 internal mini-SAS connector and 1 external mini-SAS connector, but it says it can "Supports 8 internal and 4 external SAS ports", so each connector has 4 ports and can connect to 4 drives directly, right?

2. SAS expanders

The description says it can supports up to 128 devices using SAS expanders. How does it work? If each connector has 4 port, then this controller has a total of 12 ports. How do you get to 128?

What's weird is that this LSI controller(http://www.newegg.com/Product/Product.aspx?Item=N82E16816118113) only has 2 connectors, but it claimed that it can drive up to 256 devices.

How is this possible? It has less connector than the Areca above.


3. Which SAS controller card do you recommend if I need to support 20 drives, Do I need a controller card that has 5 connectors (or 20 ports)? If not, how many connector do I need? is one enough?


4. SAS to SATA fan out cable

I have seen 1 Mini-SAS to 4 SATA and 1 Mini-SAS to 5 SATA fan out cables on the web. Any difference? Isn't a mini-SAS connector 4 port only? how can it fan out to 5 SATA?


These are some of the questions I have now. Thanks in advance for all the helps.

 

[Blue Fox]

1. Yes, each connector has 4 lanes.
2. Think of SAS expanders like USB hubs. You just wind up daisy chaining them to reach the desired number of drives.
3. If you want 20 drives and SAS, without an expander, I would recommend Areca's 1680ix-24.
4. I don't believe 5 drives is possible. Each connector only supports 4 drives. The 5th connector probably was the sideband for communicating with a backplane.

One thing that you should keep in mind. If you are not using SAS drives, SAS controllers are very picky about what SATA drives work with them.

 

[theinv]

1. Yes, each connector has 4 lanes.
2. Think of SAS expanders like USB hubs. You just wind up daisy chaining them to reach the desired number of drives.
3. If you want 20 drives and SAS, without an expander, I would recommend Areca's 1680ix-24.
4. I don't believe 5 drives is possible. Each connector only supports 4 drives. The 5th connector probably was the sideband for communicating with a backplane.

One thing that you should keep in mind. If you are not using SAS drives, SAS controllers are very picky about what SATA drives work with them.

Thanks, they are very helpful.

I have additional questions. What exactly is a SAS expander? Is it any controller that has an external SAS connector? Can you expand using internal connectors? Do they have something like SAS cable splitter so you can connect 1 internal connector to more than 4 drives?

For my setup, I will have 20 internal drives, but I would also like to have the ability to connect to more drives if necessary by using external enclosures. Controller cards with more internal connectors are expensive, will a card that has 2 internal connector and 1 external connector work for my setup?

I use only green drives from WD or Samsung, are they compatible with most SAS controllers?

 

[drozenski]

Think of a sas expander like this. You want a 128 drive array. So you would do this.

Super expensive sas card. ( Buy cheap and thats what you get )

Next you have a case with 48 drives in it and a sas expander card. The card does nothing but connect to the drives and pass data.

You then connect that card to you very expensive card. Wala you now have a huge array. But wait you wanted 128 drives right?

So you get another 48 drive case and another expander card. you plug that card in to the other card and now you have 96 drives.

It's basicaly a big dasy chain. SAS Raid card > expander > expander > expander

 

[Blue Fox]

Thanks, they are very helpful.

I have additional questions. What exactly is a SAS expander? Is it any controller that has an external SAS connector? Can you expand using internal connectors? Do they have something like SAS cable splitter so you can connect 1 internal connector to more than 4 drives?

For my setup, I will have 20 internal drives, but I would also like to have the ability to connect to more drives if necessary by using external enclosures. Controller cards with more internal connectors are expensive, will a card that has 2 internal connector and 1 external connector work for my setup?

I use only green drives from WD or Samsung, are they compatible with most SAS controllers?

Like I said, it is kinda like a USB hub, except for SAS. You can connect a SAS expander to any port, doesn't matter if it is internal or external. No way to do more than 4 drives per port as that's what SAS expanders are for.

In order to do a setup like that, you would have a SAS expander connected internally and you could also have a second connected externally.

You would have to check compatibility for each controller really to see if those drives are going to work.

Lastly, you will soon find that obtaining a SAS expander is difficult outside of ones that already come with cases. Standalone ones are rare (and about $350).

 

[Neb]

Additionaly performance isn't that great with expanders.
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