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  • 1.  jumbo frames

    Posted Oct 28, 2010 07:14 PM

    do jumbo frames really increase the performance enough to make it worth all the changes to support? does anyone have any real worth numbers comparing jumbo frames and non-jumbo frame traffic for iscsi?



  • 2.  RE: jumbo frames

    Posted Oct 28, 2010 08:07 PM

    Yes it definatally does help, so would recommend doing that. Sorry, I don't have numbers I could post.



  • 3.  RE: jumbo frames

    Posted Oct 28, 2010 08:17 PM

    What did you see to compare that jumbo frames made a difference?



  • 4.  RE: jumbo frames

    Posted Oct 28, 2010 08:39 PM

    There was defintally a speed improvement with number of reads/writes / s and number of iops. However, results do vary so its important that you do testing. We compared a VM with/without Jumbo Frames, as well as a physical server with/without Jumbo frames as well as compared it to local storage on the physical server. The setup isn't too bad to do. We did create a seperate iSCSI vSwitch with dedicated uplinks so that we could change the MTU of the vSwitch and seperate that traffic in its own VLAN.



  • 5.  RE: jumbo frames

    Posted Oct 29, 2010 04:20 AM

    They become more effecient as your load goes up.

    A SAN that is under minimal load may see a negligible difference after implementing Jumbo Frames. A SAN that is maxed out 24x7 is not going to perform significantly faster, and also significantly reduce CPU load in the process.



  • 6.  RE: jumbo frames

    Posted Oct 29, 2010 06:17 AM

    It really depends on application and the platforms. I have been doing a lot of benchmark testing lately and there are some combinations of Linux / Windows / application that perform the same, some that perform better, and some that perform worse. You just gotta test it to know. There are a lot of weird interactions here.

    For example, some of the TCP processing rules require senders to hold outbound data until fully-sized segments are ready or until timers have been triggeed, and using jumbo frames means longer hold times to fill the outbound segment. This can make chatty applications much more sluggish.

    Also if the card and TCP/IP stack are capable of processing large frames then they will often just accept them instead of dropping them. And some TCP/IP stacks probe the receiver's maximum capacity rather than limit themselves to the advertised capacity. What this means is that you may find that you are using jumbo frames in some applications even if they are not explicitly enabled in the guest OS but are enabled in the infrastructure switches.

    Test your applications and see what they are doing.