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Adjusting the TCP window size for a SnapMirror relationship

The TCP window size for SnapMirror might have an impact on SnapMirror performance. You can change the default value of the TCP window size to suit the network configuration. You can specify the window size for a particular SnapMirror relationship by modifying the corresponding entry in the snapmirror.conf file.

Before you begin

Note: You should only adjust the TCP window size for a SnapMirror relationship if there are throughput issues related to bandwidth utilization.

About this task

The TCP window size specifies the amount of data that a source can send through a connection before it requires an acknowledgement from the destination for the data received. A larger TCP window size can increase SnapMirror throughput in certain scenarios. You can change the TCP window size to optimize SnapMirror transfers for the network in use. Therefore, you can change the TCP window size to optimize SnapMirror transfers.
Note: When using higher TCP window sizes than the default, the system might not be able to achieve the maximum concurrent replication operations specified for the system. This is due to increased resource utilization by the higher TCP window sizes.
The maximum TCP window size that you can specify for a SnapMirror relationship depends on the connection type, as given in the following table.
Connection type Default TCP window size Maximum TCP window size
Single path 1,994,752 bytes 7,340,032 bytes (7 MB)
Multipath 1,994,752 bytes 14,680,064 bytes (14 MB)
Note: To limit the network bandwidth used by a particular SnapMirror relationship, use the kbs parameter for the relationship entry in the snapmirror.conf file.

Steps

  1. Calculate a TCP window size that works well for a particular SnapMirror relationship by using the following formula:

    window size = (round-trip time) × (available bandwidth)

    Example

    If the average round trip delay is 130 milliseconds and the available bandwidth is 200 Mbps, the equation is:

    window size = (((0.13 sec) × (200,000,000 bps)) / 8) bytes = 3,250,000 bytes

    Therefore, you should set the TCP window size for the SnapMirror relationship to 3,250,000 bytes.

    Example

    Similarly, you can calculate the optimal TCP window size for different round-trip time and bandwidth values. The following table provides a few examples:

    Round-trip time Available bandwidth TCP window size to maximize throughput
    120 ms 400 Mbps 6,000,000 bytes
    100 ms 1000 Mbps 12,500,000 bytes (can be used only for a multi-path SnapMirror relationship)
    50 ms 155 Mbps 968,750 bytes

  2. Specify the required TCP window size by adding the following option to the SnapMirror relationship entry in the snapmirror.conf file:
    wsize=value

    value is the required TCP window size (in bytes), as calculated in the preceding step.

    Example

    The following entry specifies a TCP window size of 3,250,000 bytes for the SnapMirror relationship:

    src_system:src_vol dst_system:dst_vol wsize=3250000 * * * *

    Example

    The following entries specify a TCP window size of 3,250,000 bytes for the multipath SnapMirror relationship:

    conxn_2=multi(10.72.146.74,10.79.106.40)(10.72.147.74,10.79.107.40)
    conxn_2:src_vol dst_system:dst_vol wsize=3250000 * * * *