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How many parallel drives, configured with RAID 0 striping (nX sequential read/write benefit) and sitting behind a hardware RAID controller (PCI-X at 133MHz) does it take to saturate a 1Gbps network connection? What about 2Gbps?

When setting up a small NAS, a big variable is the network connection. One can go with a single gigabit ethernet connection, bond several together (with managed network switches), or go with more expensive fiber channel options. Sticking to CAT5e/6 cables reduces costs to what I can manage, but I still want to get the most out of it without introducing wasteful bottlenecks.

Referring to Seagate's example of an external transfer rate (burst) of around 300MB/s (2.3Gbps) and one of storagereview.com's 3TB reviews showing sustained transfer rates of around 110MB/s (0.9Gbps), I have to come to the conclusion that there is no noticeable performance benefit to setting up RAID striping among multiple parallel drives when accessed through a 1Gbps line. A single drive uses nearly 1Gbps of bandwidth.

Of course, the data has to jump a few hoops to get from the drive controller to the cable, reducing the effective server transfer rate. Even more overhead is introduced by a RAID controller, though the parallel drives more than make up for it, but by what margin? Hence my question.

Note that a 133MHz PCI-X connector can support up to 8.5Gbps [Wikipedia]. Try to ignore the differences between stripe sizes, protocol overhead, etc., and look at the issue from a hardware perspective alone.


example answer:
My NAS motherboard 'A' (with 'B' GB of RAM) and [133MHz PCI-X] RAID controller card 'C' have a maximum throughput of about 7Gbps. The limiting factor is the ethernet controller. I have observed that a 1Gbps/2Gbps connection becomes saturated with as few as 2 drives/3 drives, at around 100MBps/200MBps using iperf.

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I have a Supermicro X8DTH-6 with an Adaptec 5405Z (PCIe) with teamed 1Gb connections. I did some experimenting to see how hard I could push my array. (4 x 250GB, 20 x 2TB) Basically I came to the conclusion that the caching on the controller would easily outrun the 2Gb worth of bandwidth for most of what we do. In this case, these boxes are used for storing security camera footage. Once the cache was exhausted on the controller, the throughput of the 4 boot disks (250GB setup as RAID10) could easily keep up with reading and writing at nearly 2Gb.. (there was some overhead that I couldn't adjust for, mostly the teaming on Layer2).

So given the limits of PCI-X 133 being 1.06GB/s being signficantly more than you are asking for, the controller would simply only be limited by the speed of the drives in the array as well as the amount of caching the controller was doing. In other words, the answer is "it depends...."

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This holds true for sequential I/O however for random I/O more spindles would be needed to reach the max transfer for a connection –  Lamar B Dec 7 '11 at 4:10
    
Perfect! So any more than 4 parallel disks wouldn't do much on a 2Gbps line. Thanks. –  tyblu Dec 8 '11 at 22:40
    
@Lamar B : I totally agree. Every instance is a bit different. Random data adds a whole level of complexity that would be difficult for the controller to compensate for. The only saving grace in that instance is to have a larger array and more paths from the controller. –  MikeAWood Dec 28 '11 at 3:13

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