Disable DelayedAck

User Guides for VMware Solutions
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DelayedAck is an advanced iSCSI option that allows or disallows an iSCSI initiator to delay acknowledgment of received data packets.

DelayedAck is highly recommended to be disabled, but is not absolutely required by Everpure. In highly-congested networks, if packets are lost, or simply take too long to be acknowledged, due to that congestion, performance can drop. If DelayedAck is enabled, where not every packet is acknowledged at once (instead one acknowledgment is sent per so many packets) far more re-transmission can occur, further exacerbating congestion. This can lead to continually decreasing performance until congestion clears. Since DelayedAck can contribute to this it is recommended to disable it in order to greatly reduce the effect of congested networks and packet retransmission.

Enabling jumbo frames can further harm this since packets that are retransmitted are far larger. If jumbo frames are enabled, it is absolutely recommended to disable DelayedAck.

See this VMware KB for more information.

Note:

BEST PRACTICE: Disable DelayedAck for FlashArray iSCSI targets.

Disabling DelayedAck:

  1. Log in to the vSphere Web Client and select the host under Hosts and Clusters.
  2. Navigate to the Configure tab.
  3. Select the Storage option.
  4. Under Storage Adapters, select the iSCSI vmhba to be modified.

    Navigate to Advanced Options and modify the DelayedAck setting by using the option that best matches your requirements, as follows:

    Option 1: Modify the DelayedAck setting on a particular discovery address (recommended) as follows:

  5. Select Targets.
  6. On a discovery address, select the Dynamic Discovery tab.
  7. Select the iSCSI server.
  8. Click Advanced.
  9. Change DelayedAck to false.

    Option 2: Modify the DelayedAck setting on a specific target as follows:

  10. Select Targets.
  11. Select the Static Discovery tab.
  12. Select the iSCSI server and click Advanced.
  13. Change DelayedAck to false.

    Option 3: Modify the DelayedAck setting globally for the iSCSI adapter as follows:

  14. Select the Advanced Options tab and click Advanced.
  15. Change DelayedAck to false.
  16. Reboot host.