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    6gb sas vs 8gb fibre

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    • bbiAngieB
      bbiAngie
      last edited by bbiAngie

      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.

      Dave.CreamerD 1 Reply Last reply Reply Quote 1
      • Deleted74295D
        Deleted74295 Banned
        last edited by

        What's it for?
        How fast do you want it to be? 🙂

        1 Reply Last reply Reply Quote 0
        • DashrenderD
          Dashrender
          last edited by

          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?

          Dave.CreamerD scottalanmillerS 2 Replies Last reply Reply Quote 0
          • Dave.CreamerD
            Dave.Creamer @Dashrender
            last edited by

            @Dashrender I would think she might mean a physical external SAS connection VS a fiber-based connection.

            bbiAngieB 1 Reply Last reply Reply Quote 1
            • bbiAngieB
              bbiAngie @Dave.Creamer
              last edited by

              @Dave.Creamer This is true.

              1 Reply Last reply Reply Quote 0
              • Dave.CreamerD
                Dave.Creamer @bbiAngie
                last edited by

                @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.

                1 Reply Last reply Reply Quote 0
                • DashrenderD
                  Dashrender
                  last edited by

                  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.

                  bbiAngieB travisdh1T Dave.CreamerD 3 Replies Last reply Reply Quote 0
                  • bbiAngieB
                    bbiAngie @Dashrender
                    last edited by

                    @Dashrender The raid/raids would be on the DAS device.

                    DashrenderD 1 Reply Last reply Reply Quote 0
                    • travisdh1T
                      travisdh1 @Dashrender
                      last edited by

                      @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.

                      1 Reply Last reply Reply Quote 0
                      • Dave.CreamerD
                        Dave.Creamer @Dashrender
                        last edited by

                        @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.

                        DashrenderD 1 Reply Last reply Reply Quote 1
                        • DashrenderD
                          Dashrender @Dave.Creamer
                          last edited by

                          @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?

                          Dave.CreamerD travisdh1T 2 Replies Last reply Reply Quote 0
                          • Dave.CreamerD
                            Dave.Creamer @Dashrender
                            last edited by

                            @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.

                            DashrenderD 2 Replies Last reply Reply Quote 0
                            • DashrenderD
                              Dashrender @bbiAngie
                              last edited by

                              @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.

                              1 Reply Last reply Reply Quote 0
                              • travisdh1T
                                travisdh1 @Dashrender
                                last edited by

                                @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.

                                1 Reply Last reply Reply Quote 1
                                • DashrenderD
                                  Dashrender @Dave.Creamer
                                  last edited by

                                  @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.

                                  1 Reply Last reply Reply Quote 0
                                  • Dave.CreamerD
                                    Dave.Creamer
                                    last edited by

                                    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.

                                    1 Reply Last reply Reply Quote 0
                                    • DashrenderD
                                      Dashrender
                                      last edited by

                                      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.

                                      bbiAngieB Dave.CreamerD travisdh1T 3 Replies Last reply Reply Quote 0
                                      • bbiAngieB
                                        bbiAngie @Dashrender
                                        last edited by

                                        @Dashrender No problem! Its all about learning. I still don't get it though....

                                        1 Reply Last reply Reply Quote 0
                                        • Dave.CreamerD
                                          Dave.Creamer @Dashrender
                                          last edited by

                                          @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.

                                          DashrenderD 1 Reply Last reply Reply Quote 0
                                          • DashrenderD
                                            Dashrender @Dave.Creamer
                                            last edited by

                                            @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.

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