2Gb Fibre Channel – Determine Maximum Throughput Speed

With storage area networks (SAN) performance issues sometimes arrise.  Knowing the maximum throughput speed of your network is key to remove it as a possible bottlekneck.  Bellow I will outline the required calculations for determining the maximum throughput speed of 2 Gigabit Fibre Channel.

Calculation Rules
Network Speeds = Always in Bits
Storage/Disk Values = Always in Bytes
Files = Always in Bytes

1 nybble = 4 bits
1 word = 16 bits
8 bits = 1 Byte
1024 Bytes = 1 Kilobyte
1024 KB = 1 Megabyte
1024 MB = 1 Gigabyte

Calculating Maximum Throughput for 2 Gigabit Fibre Channel in MegaBytes
2 Gigabits = 2/8bits = 0.25 GigaBytes
0.25 GigaBytes x 2 (2 HBAs) = 0.5 GigaBytes
0.5 x 1024 = 512 MegaBytes/sec throughput or 256 MB/sec per HBA.

Depending on the monitoring software used performance numbers may not be in MB format. Bellow I’ve broken down the values for each common throughput value.

256 MegaBytes/sec =
262,144 KiloBytes/sec
268,435,456 Bytes/sec
2,147,483,648.00 Bits/sec
33,554,432 Bits/sec

Since FC speeds have tripled since I wrote this post an update is in order. Bellow I’ve outlined speeds for 4Gb and 8Gb FC.
4Gb = 512 MB/s
8Gb = 1024 MB/s

Now that we have determined the maximum throughput of each HBA we need to rule out any bottlenecks found in the server itself or in the SAN fabric.  HBA throughput speed is directly impacted by the bus speed of each HBA as well as whether the switch ports utilized by either the storage array FA’s or the HBA itself are dedicated bandwidth or over subscribed ports.

…. brain dump in progress


11 thoughts on “2Gb Fibre Channel – Determine Maximum Throughput Speed

  1. Looks like there is an error in the last calculation. I believe it should be:

    256 MegaBytes/sec =
    262,144 KiloBytes/sec
    268,435,456 Bytes/sec
    2,147,483,648 bits/sec

  2. Hi there,

    Very nicely explained. Thanks a lot. I have one query though, I hope the thread is still open.

    I read somewhere that – Based on testing and customer experiences, any bandwidth numbers follow the “70% rule,” which dictates that any speed claimed by the hardware vendor be reduced to 70% of the top-rated speed. For instance, 1 Gigabit Ethernet (GbE) speed is theoretically 125 MB/sec, it would be – (125 * 70% = 87.5 MB/sec) practically. My question is, does it also apply to Fibre-channels. If so, will these figures change :

    For 4Gb and 8Gb FC with 70% rule.
    4Gb = 512 MB/s = 358 MB/s
    8Gb = 1024 MB/s = 716 MB/s

    Thanks in advance!

  3. Excellent question. To be quite honest I’m not sure whether there is a similar overhead ratio with Fibre Channel(FC) networks. In most high volume FC networks I’ve worked in the actual fabric isn’t the bottleneck. In some cases the physical server hardware is a constraint. An example would be using a PCI vs. PCI-Express slot which would give you a limit of 133MB/s vs. 256MB/s at the motherboard layer. It’s also important to keep in mind the port constraints of the fabric. Depending on the switch vendor and the size of your fabric different over subscription ratios may exist. An example would be if a group of say 6 ports share 12Gb/s of bandwidth. If each of the 6 ports pushes as much I/O as possible at the same time each port will max out at a theoretical 2Gb/s. If each host thinks it has a 4Gb/s FC link then you can quickly see where a potential bottleneck lies.

    You can see how the specific environment components can make a huge difference when looking at the “maximum through put” of an FC link. And this isn’t even approaching application layer inefficiencies. One last piece to add. In order to really figure out whether you have a real bottleneck within your FC network setup a tool which polls FC port throughput details with a high frequency. Say once every 5 seconds. Collect the data for a day, week, or even a month and then import it into Excel and chart the frequency of bandwidth utilization. Use of histograms and pivot tables make this task easier. The frequency of spikes dictates the utilization of bandwidth. Based on the calculations I’ve already presented you can also gauge percent utilization etc.

  4. Thanks a ton for explaining so well. I have noted down your suggestions, and whenever I get an opportunity to test it, I will certainly do that. Very informational blog, keep sharing knowledge! You are doing a great job.

  5. It doesn’t look like these calculation include the 8b/10b encoding, I think you have to take 80% off the top to account for this. For 10 bits of transmitted data, only 8 bits are data.

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