vSAN Health Service - Physical Disk Health

Products

VMware vSAN

Issue/Introduction

This article explains the Physical Disk Health - Memory pools (heaps) and the Physical Disk Health - Memory pools (slabs) check in the vSAN Health Service and provides details on why it might report an error.

Environment

VMware vSAN 6.5.x
VMware vSAN 6.7.x
VMware vSAN 6.6.x
VMware vSAN 8.0.x
VMware vSAN 6.2.x
VMware vSAN 7.0.x

Resolution

Q: What does the Physical Disk Health – Memory pools check do?

This health check is responsible for checking the memory pools used by vSAN and reports if they are running low. Physical disks have a requirement on adequate memory when used with vSAN.

Q: What does it mean when it is in an error state?

If this health check is not green (OK), it indicates that vSAN is running low on a vital memory pool needed for the operation of the physical disks.

The threshold for these two health checks are as the following:

Memory pools (slabs)

Red: free slabs < 5% for any disk in any host of this cluster
Yellow: free slabs >= 5% while < 25% for any disk in any host of this cluster
Green: free slabs >= 25% for all disks in this cluster

Memory pools (heaps)

Red: free heap < 5% for any disk in any host of this cluster
Yellow: free heap >= 5% while < 15% for any disk in any host of this cluster
Green: free heap >= 15% for all disks in this cluster

When under load, vSAN uses congestion as a means to throttle down the incoming I/O rate in order to relieve pressure on memory pools and keep them at safe levels. Because of this safeguard, memory pool depletion should not happen.

It is highly likely that the physical disk will also report a congestion health error.

This can lead to a variety of performance issues such as virtual machine storage performance degradation, operation failures, or even ESXi hosts going unresponsive.

Hybrid environments with a high workload running on vSAN coupled with a large number of concurrent read cache misses may result in memory pool pressures.

Note: If this health check is not OK, it will typically not be as a result of any issue with the hardware or software but simply the result of a genuine high load running on vSAN.

Q: How does one troubleshoot and fix the error state?

You can attempt to place the host into Maintenance Mode with Ensure Accessibility and monitor for improvement to the vSAN health check results.

If this health check persists and does not improve, contact VMware Support. For more information, see How to file a Support Request in Customer Connect (2006985).

This behavior is improved in later 6.5 releases.

Additional Information

For more information on collecting VMware vSAN logs, see Collecting vSAN support logs and uploading to VMware (2072796).

Also, see:

Documentation: