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I'm going to buy an 8-port raid controller for which the declared data trasfer rate is "Up to 6Gb/s per port". The declared connectors are "Two mini-SAS SFF8088". The maximum number of supported physical devices is "Up to 32". The controller is Dell PERC H710.

I'm not sure about the maximum bandwidth I should expect. The server backplane supports 8 SSD slots, so I will not reach the maximum of 32 disks but the maximum of 8 disks.

Supposing each disk able to provide 6Gbps for sequential trasfers, should I expect a maximum bandwidth of 48Gbps, or 6Gbps per mini-SAS (i.e. 12Gbps)?

Also, if the maximum number of physical disks supported is 32, how the bandwidth will distribute between drives, always supposing 6Gbps capable drives? The 48Gbps total bandwidth will be distributed on 32 drives (32 x 1.5Gbps) as on 8 drives (8 x 6Gbps)?

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That particular RAID controller claims 8-lane PCI Express 2.0 compliancy meaning that effectively you'll already be limited to 8 * 4 Gbs = 32 Gbs (or 4000 MB/s) regardless of what's connected to the RAID card.

Each SAS SFF8088 connector will cary 4 SAS lanes over a single cable, when each link is at the maximum 6 Gbs port speed theoretically you indeed get 2 x 4 x 6 Gbs = 48 Gbs worth of bandwidth that the RAID controller can manage.

A 6 Gbs SAS link will be shared by the number of devices connected to it, so if you connect 4 devices and each is stressed equally you can only get 1,5 Gbs per individual drive.

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Based on my experience with similar controllers like Smart Array P420i from ProLiant ML350p Gen8 or ServeRAID M5015 SAS/SATA Controller from IBM x3560 M3 I'd say: 6Gbps per mini-SAS (i.e. 12Gbps) - best possible scenario. I have not tested two SFF8088 ports, only one, and I've hit 6 gbps limit.

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