TCP-delayed Ack delays an acknowledgment for a received packet for the ESXi host.

So certainly, TCP-delayed Ack should be disabled on the iSCSI network used for storage.

If you have this feature enabled, then an acknowledgment for a packet is delayed up to 0.5 seconds or until two packets are received. Sending an acknowledgment after 0.5 seconds is a sign that the network is congested. Because there was no communication between the host computer and the storage system during those 0.5 seconds, the host computer issues inquiry commands to the storage system for all LUNs based on the delayed Ack. When there is congestion in the network and recovery of dropped packets, delayed Ack can slow down the recovery considerably, resulting in further performance degradation.

To implement this workaround in ESXi 6.5, use vSphere Web Client to disable the delayed ACK, as follows:

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

5. Modify the delayed Ack setting, using the option that best matches your site's needs, as follows:
   • Modify the delayed Ack setting on a discovery address (recommended) as follows:
     1. Click on Dynamic Discovery on the Targets tab.
     2. Click on the iSCSI server, then click on Advanced.
     3. Find the DelayedAck option.
     4. Uncheck Enabled in the Inherited column, then uncheck Enabled in the Value column.
     5. Click on OK.
   • Modify the delayed Ack setting on a specific target, as follows:
     1. Click on Static Discovery on the Targets tab.

     2. Click on the iSCSI server, then click on Advanced.

     3. Find the DelayedAck option.
     4. Uncheck Enabled in the Inherited column, then uncheck Enabled in the Value column.
     5. Click on OK.
   • Modify the delayed Ack setting globally, as follows:
     1. Click on the Advanced Options tab.
     2. Click on Edit.
     3. Find the DelayedAck option.
     4. Uncheck Enabled in the Value column.
     5. Click on OK.
6. Reboot the host.