How to set proper TCP buffer size?

305 pts.
Tags:
AS/400
MIMIX
TCP
Hi, We have 3 AS/400 systems PR, HA (Both are on same physical location) & DR (situated in different physical location around 1000 KM away from PR system). All the three systems are in sync using MIMIX Global. We had 100 mbps WAN link configured for PR to DR replication. PR & HA replication is on internal network. We are using remote journals for MIMIX. We have started facing RJ lag during night time when there are huge journal entries getting created due to End of the Day jobs. This Remote Journal lag goes beyond 30 min. & 15 million transactions on few days. Hence management has decided to increase the network bandwidth & not we are on 200 mbps WAN link. Still no improvement in network performance. We are not having any replication problem between PR & HA as these systems are in same data center connected on local LAN. As confirmed by our network team iSeries system are not utilizing the full bandwidth and bandwidth utilization is only 40%. We are not having any RJ lag during normal business hours. We have checked Retransmissions but they are not much. MTU value also set to *IFC. The questions are, 1) Will increasing/decreasing the TCP Buffer Size on source & target systems help us to improve network performance? & 2) How to identify the exact TCP Buffer size required for best network performance? . -Ketan

Software/Hardware used:
AS400/IBM i OS V7R1

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  • Ketan19
    A Typo........Please read it as& now we are on 200 mbps WAN link..-Ketan
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  • TomLiotta
    ...iSeries system are not utilizing the full bandwidth and bandwidth utilization is only 40%.   Why? That is, if your network team can give a bandwidth utilization figure, did they explain what was needed in order to improve that figure? Is the cause of a low utilization determined to be on PR, DR or both? A TCP buffer size would possibly be changed if there was some specific evidence from your network team that a change could help, but what was the evidence?   In particular, why is "buffer size" thought to have any relationship to the problem?   See the CHGTCPA command for a general method of setting buffer size. The TCPRCVBUF() and TCPSNDBUF() parms are relevant depending on whether you're looking at the PR or the DR system.   But before making any changes at all, you should ask MIMIX what they do with the buffer size for their connections. An application can set the buffer size that it wants, and any change that you make can be ignored.   After that, if MIMIX uses the TCP setting, you should track down why there is a thought that buffer size has a relationship to bandwidth utilization in your case. I might suspect other elements first.   Tom
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  • Ketan19
    Hi,Network team has given the bandwidth utilization logs showing less than 40% utilization of bandwidth & they are saying nothing is wrong from network side. The remote journals are transferred from PR to DR normally (Except during DR drill, when Source system is DR). The low bandwidth utilization remains same whether my source system is PR or DR..We are having same network configuration (TCP Buffer size & Link speed, etc..) on all the systems..MIMIX is not interfering with Remote Journal transfer & only performs  Receiver management on Source & Target systems..Rest of the things looks fine from iSeries side. Thus we thought changing TCP buffer size might help in this case..On iSeries, is there a way to identify how much buffer size is required for TCP Send & Receive when data transfer is going on. If we have theexact figures this will help to address this problem..Note: We are transferring aprox. 60 GB SAVF from PR to DR using MIMIX (SNDNETOBJ) daily. This file was used to take approx. 3.5 hrs on 100 mbps link. On 200 mbps link also it is taking the same amount of time. Ideally the time should be lesser after increasing the bandwidth.Thus questions are now pointing towards iseries instead of network & we need to identify the cause of this problem..-Ketan
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  • TomLiotta
    So, what did MIMIX Support tell you about how their product handles buffer size? Without their comments, there's no reason to think that you can affect anything. And any change that you make can just as easily make things worse. -- Tom
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  • Ketan19
    Hi,
    We had opened incident initially with Vision Solutions but they said as there is no lag in MIMIX apply sessions and it is Remote Journal Lag, Vision Solutions cannot support this issue. We are currently working with IBM to resolve this problem.
    .
    I will post the resolution of this, once we will come up with definite solution.
    Thanks for the discussion.
    .
    -Ketan

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  • TomLiotta

    Okay, then MIMIX is using the system default buffer sizes. You can adjust the defaults with the TCPRCVBUF() and TCPSNDBUF() parms of the CHGTCPA command.

    The (default) buffer sizes for sending and receiving can be adjusted on both PR and DR. It will take some experimenting to determine what sizes are appropriate on each end of the connection. You might also want to watch for any issues for other applications, not just MIMIX.

    The buffer size isn't quite directly related to bandwidth. Read the help text for both parameters to be sure you understand what the buffers are for.

    Tom

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