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Slow VMware Consolidated Backup performance with software iSCSI initiator

Symptoms

You may experience these symptoms:
  • Slow performance while using vcbMounter.exe
  • The SAN adapter shows only 1% resource usage

Resolution

Modify the TCP/IP settings for the network interfaces carrying iSCSI traffic to immediately acknowledge incoming TCP segments.

Note: This workaround solves the read performance issue.
On a server that runs Windows Server 2003 SP1 or later:

  1. Start Registry Editor (Regedit.exe).
  2. Locate and click the following registry subkey:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces

  3. The interfaces are listed underneath by automatically generated GUIDs like, {064A622F-850B-4C97-96B3-0F0E99162E56}.

    Click each of the interface GUIDs and:
    1. Check the IP Address or DHCP IP Address parameters to determine whether the interface is used for iSCSI traffic. If not, skip to the next interface.
    2. Click Edit > New > DWORD value.
    3. Name the new value TcpAckFrequency, and assign it a value of 1.
    4. Exit the Registry Editor.
    5. Restart Windows for this change to take effect.

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