Gigabit Speed, How fast is it really?

Discussion in 'Networking, Telephony & Internet' started by tylerplowright, Jul 26, 2010.

  1. tylerplowright

    tylerplowright Member

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    Hi Guys,
    had a question about transfers of files over gigabit. Motherboard has a gigabit ethernet port, and other computer has a pci gigabit card. Both computer have raid setups so there is no worries about hard drives slowing the speed down.

    Computer will be connected via cat 5e crossover cable that's about a meter long.

    How fast would it REALLY transfer at. Files are like 500mb in size on average. Will need to transfer about a gigabyte worth of data.

    Any ideas, am just curious.

    Tyler
     
  2. terrastrife

    terrastrife Member

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    dependinng on the cards usually you should see about 75MB-125MB/sec

    so 5 seconds.
     
  3. LostBenji

    LostBenji Member

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    Yup, the usual slowdown culprit is the OS and drive I/O control. RAID doesn't always mean high speeds, especially if the OS is holding you back.
     
  4. caspian

    caspian Member

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    in his case, the PCI bus is not helping either.
     
  5. OP
    OP
    tylerplowright

    tylerplowright Member

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    Figured it would be the pci that limited things. There are no pci-e slots on the other mobo (its a bit old and is only being used as a tv computer, saves dvd's).

    But 75/125mb could be right...idk spose i'll have to wait and see. Either way i'm predicting like 5 hours just to be safe.
     
  6. ir0nhide

    ir0nhide Member

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    5 hours for a gigabyte? Plain old 100Mbps networks can generally do 9-10Mbytes/sec, which means the entire transfer is done in less than 2 mins.
     
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    tylerplowright

    tylerplowright Member

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    Average file size is 500mb, total size of everything i want to copy is like 1tb, hence the 5 hours.

    Reading back on first post that should have said terabyte, not gigabyte
     
  8. Hive

    Hive Member

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    You will never see higher than ~100MB/s due to protocol overhead
     
    Last edited: Jul 27, 2010
  9. ltd73

    ltd73 Member

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    In theory there is no difference between theory and practice. But, in practice, you're wrong.
     
  10. Hive

    Hive Member

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    Ok prove me wrong

    Ever heard of overhead? hmm?

    In theory USB 2.0 can move 480Mbit/s

    In reality you only get half that
     
  11. vkritter

    vkritter Member

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    Code:
    [dump@node0212 ~]# wget http://10.3.1.10/centos/5.4/isos/x86_64/CentOS-5.4-x86_64-bin-DVD.iso
    --2010-07-27 16:28:18--  http://10.3.1.10/centos/5.4/isos/x86_64/CentOS-5.4-x86_64-bin-DVD.iso
    Connecting to 10.3.1.10:80... connected.
    HTTP request sent, awaiting response... 200 OK
    Length: 4619442176 (4.3G) [application/octet-stream]
    Saving to: `CentOS-5.4-x86_64-bin-DVD.iso.1'
    
     9% [==>                                    ] 451,072,000  115M/s  eta 38s
    workstation is the bottleneck, 'only' has gigabit.

    Potential Overheads being QOS and two switches between the server and client.
     
  12. ltd73

    ltd73 Member

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    okay.

    yep! know all about it!

    ok, time for the math/theory.

    layer 1:
    Gigabit Ethernet is 1250 Mb/s at the physical layer. encoding at 8b/10 results in 1000 Mbit/s at the data link layer, 8 bits/byte gets you to 125000000 bytes/sec or 125MB/s where MB is power-of-ten based.

    lets take it up to layer 2:
    lets assume to start with that we're working with reasonable frame sizes. this will be the case for a bulk file transfer so lets assume the majority of traffic will be at 1500 byte 'payload'. note that jumbo frames will reduce this overhead slightly, allowing for potentially 9216 bytes payload for the same header size... but lets assume non-jumbo case.

    for IEEE 802.3 format with 1500 byte payload, we can get 82671.96 frames/second on the wire before that wire has no more capacity (125MB/s).

    take this up to layer 3:
    each IP packet at layer 3 has a 20 byte IP header.

    take this up to layer 4:
    data will typically be transferred using TCP.
    each TCP segment will have a 20 byte TCP header on it.

    ok, lets now work the sums backwards. for each of 82671.96 packets/second we can get (1500-(20+20)) = 1460 bytes of 'data' into it.

    1460 bytes x 82671.96pps = 120701061.60 bytes/second = 120.70106MB/s.

    overhead is ~3.44%. well short of the 20+% you state.


    thats the theory. i can show you iSCSI getting exactly that. i can show you FTP getting exactly that. hell, i can even show you 10GbE getting _exactly_ 10x that value.

    actually, truth be told, i can show you something getting 100.01% of that rate due to oscillator clock jitter tolerances of 100ppm given the right two ethernet ports.

    just for grins and giggles lets say that 9000 byte jumbo frames were used.
    this will mean that the 'wire' for gigabit is capable of up to 13870.40 packets/sec for a 9000 byte 'payload'.
    we still have the same IP + TCP overhead of 20 bytes each so we end up with (9000 - (20+20) bytes) x 13870.40 packets/sec = 124.278MB/s.
    OR, put another way, for a 'wire' capable of 125MB/s that overhead is 0.578%.

    well short of the ~20% overhead you imply.

    actually, "reality" is closer to 2/3rds that for 'high speed devices' but whose counting?
     
    Last edited: Jul 27, 2010
  13. Soarer GT

    Soarer GT Member

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    I thought 9k jumbo frames would transfer faster than 125MB/s ? is there a benefit to using Jumbo Frames other than the standard 1500MTU given you have enough capacity with IO speed? (ie raid 10) ?
     
  14. Heywood

    Heywood Member

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    One of the advantages of jumbo frame support is less interrupts to the CPU assuming you're process switching
     
  15. bumography

    bumography Member

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    I'm seeing around 62.5 megabytes/s transfers of big files.
    But then my hard drives might be limiting factors.
     
  16. cs-cam

    cs-cam Member

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    ITS A TRAP
     
  17. Aetherone

    Aetherone Member

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    Is this the sort of thing you were looking for?

    [​IMG]

    Welcome to 2006 and Pentium4!
     
  18. kripz

    kripz Member

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    The only comeback he has left after that beating is "lol trolled".

    One thing that pisses me off is Samba overheads, it seems higher than 2 Windows pc transferring to each other.
     
  19. OP
    OP
    tylerplowright

    tylerplowright Member

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    ok i thought i could keep up with this, but now its gone well and truly over my head. Doesn't matter, I know what it could do, it will just depend on that PCI i think. Will post how fast it does go when I copy stuff in a couple of days.

    Any easy way to check how fast it is actually copying at on XP?
     
  20. the_pessimist

    the_pessimist Member

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    yeah task manager.
     

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