6gb sas vs 8gb fibre
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Hey everyone,
I am needing some technical details on which one is better for DAS connectivity. I was having a conversation with a coworker but due to a lack in knowledge, we cannot come to a decision. Now pretend that pricing has ZERO factor. I want to know the technical nitty gritty details as to why you would choose one over the other. Assume the setup is a DAS device sitting in between 2 servers. -
What's it for?
How fast do you want it to be?
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Can you even compare these two items?
6Gb SAS is the speed of the drives
8 Gb fibre is the connection speed between the host and the DAS chassis.
Right?
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@Dashrender I would think she might mean a physical external SAS connection VS a fiber-based connection.
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@Dave.Creamer This is true.
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@bbiAngie In the experience I have, it comes down to features. I'm not saying anything bad about either, but a SAS connection would be directly attached to a server, where as fiber would typically go through some sort of SAN switch. Typically. The growth on the fiber, with the switch is a plus - depending on the environment. We run both with different clients, as well as internally for all sorts of different reasons. We use SAS as a direct attached backup storage system, but fiber for all of our VM SAN storage.
Now, I know there are others that will jump in as well. I am but one voice in the sea of many.
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So I guess the next question is - where is the RAID taking place?
As I understand it, if you have an external DAS and are using SAS to connect to the RAID controller directly, then you have direct access to every drive at full speed, just like you would in the chassis.
Assuming 10 drives, at 6 Gb each - you should get, at max, 60 Gb of throughput.If fibre also connects to a RAID controller in the server, IF it works like SAS, then the 8 Gb per drive is just extra that is wasted. But if their is only a single 8 Gb connection, then instead of being at 60 max, you're majorly reduced to 8 Gb.
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@Dashrender The raid/raids would be on the DAS device.
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@Dashrender said:
So I guess the next question is - where is the RAID taking place?
As I understand it, if you have an external DAS and are using SAS to connect to the RAID controller directly, then you have direct access to every drive at full speed, just like you would in the chassis.
Assuming 10 drives, at 6 Gb each - you should get, at max, 60 Gb of throughput.None of the SAS connected DAS devices I've seen work like that. It's normally all connected on one or two SAS channels. Course that's all low end stuff, I imagine the higher end devices may be different.
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@Dashrender Hmm...I may be wrong here, but 6Gb is 6Gb. If you have a 6Gb SAS connection, the max you'll get for throughput is just that - 6Gb.
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@Dave.Creamer said:
@Dashrender Hmm...I may be wrong here, but 6Gb is 6Gb. If you have a 6Gb SAS connection, the max you'll get for throughput is just that - 6Gb.
Actually I am not sure how that works.
Each drive is rated at 6 Gb/s - right?Assuming that, two drives running in RAID 0 should be able to pump data out at 12 Gb/s.
Is that incorrect?
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@Dashrender Not if the physical connection to the host server is only capable of 6Gb throughput. It's not local storage, so like any attached storage it's limited to the speed of the adapter.
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@bbiAngie said:
@Dashrender The raid/raids would be on the DAS device.
Sure the arrays would be in the DAS, but the controller?
Granted I haven't looked at a system like this in over 15 years (I've been completely small business during this time) - but when I did last participate in building a system like this, the controllers where in the main server, and only disk were in the DAS chassis.
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@Dashrender said:
@Dave.Creamer said:
@Dashrender Hmm...I may be wrong here, but 6Gb is 6Gb. If you have a 6Gb SAS connection, the max you'll get for throughput is just that - 6Gb.
Actually I am not sure how that works.
Each drive is rated at 6 Gb/s - right?Assuming that, two drives running in RAID 0 should be able to pump data out at 12 Gb/s.
Is that incorrect?
DAS units normally have a back plane of some sort. So you are really hooking 4 to 8 drives to a single SAS channel. Spinning rust this doesn't matter so much as current SAS/SATA standards are so much faster than the drives can work with data... start dropping SSD into a SAS attached DAS and you could cause yourself a bottleneck.
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@Dave.Creamer said:
@Dashrender Not if the physical connection to the host server is only capable of 6Gb throughput. It's not local storage, so like any attached storage it's limited to the speed of the adapter.
Definitely - so RAID controllers are limited to 6 Gb? I always wondered how that worked.
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It may depend on the system then. We just rolled out a few SAS connected IBM SA120 devices. Mid-low end, but decent. The SAS card in the host server is only a 6Gb card, so throughput is limited to that speed. Now, the drives are all 6Gb drives, so they will run at max speed, with a little lag due to read/writes at the same time, and a few other operations.
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huh - cool - looks like I might have been thinking about it all wrong.
Sorry I'm possibly putting some bad info into your thread @bbiAngie at least I'm learning some cool stuff along the way.
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@Dashrender No problem! Its all about learning. I still don't get it though....
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@Dashrender Hey - I'm waiting for someone to pop in and smack me around any second. But until they do...
Just kidding. Like I said, they're limited to the connection speed of the card, the chassis and the drives.
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@Dave.Creamer said:
@Dashrender Not if the physical connection to the host server is only capable of 6Gb throughput. It's not local storage, so like any attached storage it's limited to the speed of the adapter.
Why would local or not local (DAS is kinda like local though).
The backplane in DAS is just like a backplane inside the server. Typically two backplanes, each with a cable to the controller.
If what Dave says is correct (and now I'm inclined to think he is) either those channels, or the whole card could be limited to 6 Gb.
But it wouldn't matter internal or DAS, they would both be the same speed.
