IBM Support

Host Disconnects Using VMware vSphere 5.5 Update 2, 5.5 Update 3, 6.0, 6.0 Update 1 and 6.0 Update 2

Flashes (Alerts)


VMware vSphere 5.5 Update 2, 5.5 Update 3, 6.0, 6.0 Update 1 and 6.0 Update 2 hosts may experience multiple repeated disconnects leading to application outages in response to delays in completing heartbeat I/O.


A change was introduced in VMware vSphere 5.5 Update 2 and vSphere 6.0 to use Atomic Test and Set (ATS) by default for heartbeat I/O.

The use of ATS heartbeating is now supported on IBM SAN Volume Controller, Storwize and Spectrum Virtualize systems running 7.6.1.x code or later, with VMware vSphere environments using ESXi 6.0u3 or later. However, ATS Heartbeating is still not supported on VMware vSphere ESXi 5.5 environments. Please note that "ATS Heartbeats" and "ATS for Hardware Accelerated Locking" are two separate settings. The issue reported by this flash only applies to ATS Heartbeats.

To enable ATS heartbeating on a particular host, use the following command


  • esxcli system settings advanced set -i 1 -o /VMFS3/useATSForHBOnVMFS5

Due to the low timeout value for heartbeat I/O using ATS, this can lead to host disconnects and application outages if delays of 8 seconds or longer are experienced in completing individual heartbeat I/Os on backend storage systems or the SAN infrastructure.


Release 7.6.0 introduced enhancements to the way that SVC and Storwize process ATS, which will reduce the likelihood of experiencing this issue.

To prevent VMware hosts on these levels from experiencing this issue, the following commands can be issued to disable using ATS for heartbeat I/O and revert to the 5.5 Update 1 heartbeat method. Note that after issuing these commands, the host will NOT be required to be rebooted.


  • esxcli system settings advanced set -i 0 -o /VMFS3/useATSForHBOnVMFS3

(N.B. ATS for heartbeat is disabled by default with VMFS-3, so this command will only need to be issued if it has previously been manually enabled)



  • esxcli system settings advanced set -i 0 -o /VMFS3/useATSForHBOnVMFS5

Refer to the following VMware Knowledge Base article for more details on how to query and modify these settings:

For any queries about the above commands, contact VMware support.

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Document Information

Modified date:
20 March 2019